Q: In Heb, what are early New Testament manuscripts we have preserved today?
A: Here are many of them.
p12 (285-300 A.D.) Hebrews 1:1
End of 3rd century - 1968 - The Text of the New Testament.
Late 3rd century - 1998 - Aland et al. Fourth Revised Edition
p13 (=Papyrus Oxyrhynchus 657) contains Hebrews 2:14-5:5; 10:8-22; 10:29-11:13; 11:28b-12:17 (c.225-250 A.D.) A photograph of part of this is in The Complete Text of the Earliest New Testament Manuscripts p.74. It says on p.75 that out of 88 variation units, p13 and p46 has 71 agreements and only 17 disagreements.
4th century - 1902-1903 - Grenfell and Hunt according to The Complete Text of the Earliest New Testament Manuscripts p.73. On p.17 it says Grenfell and Hunt hesitated to date ANY New Testament manuscript earlier than the third century, even though they mentioned the handwriting belonged to the late first or second century.
3rd/4th century, perhaps first half of fourth century - 1968 - The Text of the New Testament also has 11:14-28a, but The Complete Text of the Earliest New Testament Manuscripts p.74 says this is missing due to the loss of one column.
3rd/4th century - 1975 - Aland et al. Third Edition
3rd/4th century - 1998 - Aland et al. Fourth Revised Edition
200-250 A.D. - others
(225-250 A.D.) - 1999 - The Complete Text of the Earliest New Testament Manuscripts p.73. Comfort and Barrett on p.73 say that based on very similar handwriting to Papyrus Oxyrhynchus 852 (175-25 A.D.), many similarities to Papyrus Oxyrhynchus 2635 (ca. 200 A.D.). The dating is not earlier than 225 A.D. though, because it is written on the back side of Papyrus 657 (part of the Epitome of Livy’s History of Rome), and this is dated to the third century.
p17 Heb 9:12-19 (late 3rd century)
4th century - 1968 - The Text of the New Testament.
p46 Chester Beatty II ca.200 A.D. 300 of the 303 verses of Hebrews. Specifically it has Hebrews 1:1-9:16; 9:18-10:10; 10:22-30; 10:32-13:25 (290 verses) and other parts of Paul’s letters. The Complete Text of the Earliest New Testament Manuscripts has a photograph of part of p46 on p.192. It also says on p.197-198 that the quality and the stichiometric marks show that a professional scribe wrote this.
c.200 A.D. - 1968 - The Text of the New Testament.
About 200 A.D. - 1975 - Aland et al. Third Edition
About 200 A.D. - 1998 - Aland et al. Fourth Revised Edition
p79 Hebrews 10:10-12,28-30 (7th century)
p89 Hebrews 6:7-9,15-17 (4th century)
p114 (=Papyrus Oxyrhynchus 4498) Hebrews 1:7-12 (3rd century)
p116 (=Papyrus Vindob. G42417) Hebrews 2:9-11; 3:3-6 (6th century)
Cyril of Jerusalem 386 A.D. Chrysostom 407 A.D.
Vaticanus (325-350 A.D.) contains all of Hebrews except Hebrews 9:14-13:25 according to Gospel Parallels.
Sinaiticus (340-350 A.D.) and Alexandrinus (c.450 A.D.) contain all of Hebrews.
Bohairic Coptic 3rd/4th
Fayyumic Coptic 3rd/4th century
Sahidic Coptic 3rd/4rth century
Palestinian Syriac [Syr Pal] from c.6th century
Peshitta Syriac [Syr P] 411-435 A.D... Over 350 manuscripts.
Harclean Syriac [Syr H] 616.A.D Thomas of Harkel
Ephraemi Rescriptus 5th century
Armenian [Arm] from 5th century
Georgian [Geo] from 5th century
Bezae Cantabrigiensis 5th or 6th century
Washington 5th century
Ethiopic [Eth] from c.500 A.D.
Wolfenbuttel 6th century

Q: In Heb, what are the manuscript variations with the Textus Receptus, the basis for the KJV?
A: Jay P. Green, Sr. in the Interlinear Bible records variations in approximately 87 words between the Textus Receptus and the majority text. These are in 55 places. He does not record any additional alternates.

Q: In Heb, what are some of the manuscript variations?
A: The book of Hebrews has a total of about 4,888 Greek words and a word-for-word accuracy of 98.3%, with only 84 words in question. These are in 34 verses (47 places), out of 303 total verses. Of the 44 places, 38 are single word. Below are the variations with the primary choice and the top alternate choice. Aland, from which this primarily is based, also gives a judgment of the degree of certainty for each variation.
Heb 1:12 "you shall roll them up" (Chester Beatty II, second corrector of Sinaiticus, Vaticanus, second corrector of Bezae Cantabrigiensis, Byzantine Lectionary, Chrysostom, many others) vs. "you shall change" (original Bezae Cantabrigiensis, Athanasius) (1 word, not counted in the totals)
Heb 1:3 "him cleansing" vs. "him through/by himself cleansing" (2 words)
Heb 1:8 "of you/your" vs. "of him/his" vs. (absent)
Heb 1:12 "as a garment/clothing and" vs. "and as a garment/clothing" vs. "and"
Heb 2:7 "crowned him" (p46, Vaticanus, Bezae Cantabrigiensis, some Byzantine lectionary) vs. "crowned him and set him over the works of your hands" (Sinaiticus, Alexandrinus, some Byzantine Lectionary) (10 words) (This matches the phrase in Psalm 8:6)
Heb 2:9 "by the grace of God" vs. "without which/besides God" (3 letters) (Almost all early manuscripts in the Alexandrian and Byzantine family support the first. No one supports the second, except Origen and Ambrose and the NRSV footnote. Thus, it is rather certain it is the first reading and is not counted in the totals.)
Heb 3:2 "in all the house" vs. "in the house"
Heb 3:6 "where" vs. "who"
Heb 3:6 "until/to the end firm we hold fast" vs. "we hold fast" (2 words)
Heb 4:2 "to mix/unite with" vs. "having been mixed/united with"
Heb 4:3 "For" vs. "Therefore"
Heb 4:3 "into the rest" (almost all other early manuscripts, Byzantine Lectionary) vs. "into rest" (p13, Chester Beatty II, Vaticanus, original Bezae Cantabrigiensis)
Heb 4:3 "we enter" vs. "let us enter"
Heb 5:11 "about this" vs. "about Him"
Heb 5:12 "placement of accent mark on the word "someone"
Heb 6:2 "baptism teachings/doctrine" vs. "teachings/doctrine of baptism"
Heb 6:3 "we will do" vs. "let us do" (1 letter)
Heb 7:4 "Even Abraham" vs. "Abraham"
Heb 7:21 "priest forever" (p46, Vaticanus, Ephraemi Rescriptus, Sahidic Coptic) vs. "priest forever according to the order of Melchizedek" (second corrector of Sinaiticus, Alexandrinus, Bezae Cantabrigiensis, Byzantine Lectionary, Chrysostom) vs. (absent) (original Sinaiticus) (7 words)
Heb 8:8 "fault with the people" vs. "fault and said to the people"
Heb 8:11 "fellow man/citizen" vs. "neighbor"
Heb 9:1 "therefore also/and" (Sinaiticus, Alexandrinus, Bezae Cantabrigiensis, some Byzantine Lectionary, Chrysostom) vs. "therefore" (apparent p46, Vaticanus, Sahidic Coptic, Bohairic Coptic, Fayyumic Coptic, some Byzantine Lectionary)
Heb 9:10 "ordinances" vs. "and ordinances"
Heb 9:11 "good things that have come" vs. "good things that will come"
Heb 9:14 "eternal" (apparent p17, original Sinaiticus, Vaticanus, Alexandrinus, second corrector of Bezae Cantabrigiensis, Byzantine Lectionary) vs. "holy" (second corrector of Sinaiticus, original Bezae Cantabrigiensis, Bohairic Coptic, Fayyumic Coptic, Chrysostom)
Heb 9:14 "our" vs. "your" (plural)
Heb 9:14 "eternal spirit" vs. "Holy Spirit"
Heb 9:17 "for it is then… lives." vs. "for is it then…lives?"
Heb 9:17 "in no way" (second corrector of Sinaiticus, Alexandrinus, Ephraemi Rescriptus, second corrector of Bezae Cantabrigiensis, Byzantine Lectionary) vs. "not ever" (original Sinaiticus, original Bezae Cantabrigiensis, Ethiopic)
Heb 9:19 "calves and the goats" vs. "goats and the calves" vs. "calves and goats" vs. "calves" (2 words)
Heb 10:1 "not itself" vs. "and" (2 words)
Heb 10:1 "it" vs. "they"
Heb 10:1 "it never is able" vs. "they never are able"
Heb 10:11 "priest" vs. "high priest"
Heb 10:34 "bonds" (same meaning and same grammar)
Heb 10:34 "yourselves" vs. "in yourselves"
Heb 10:38 "my just by faith" vs. "just by my faith" vs. "just by faith"
Heb 11:1 "assurance of things" (p46, almost all early manuscripts, Byzantine Lectionary) vs. "of things of resurrection" (p13, Origen, so not counted in the totals)
Heb 11:4 "God" vs. (absent) (All the main manuscripts have "God" except for p13 (3rd/4th century) and Clement of Alexandria)
Heb 11:4 "the" vs. "the/this"
Heb 11:11 "Sarah barren" (p46, Bezae Cantabrigiensis) vs. "Sarah" (Sinaiticus, Alexandrinus, correct Bezae Cantabrigiensis, Byzantine Lectionary, Chrysostom) vs. "Sarah herself, though barren, received power to conceive, even when she was too old, because she considered him faithful who had promised" (only Sahidic Coptic) (approximately 15 words not counted in the totals)
Heb 11:23 "king." vs. "king. By great faith, was born Moses did not forebear to kill the Egyptian when he saw the humiliation of his brothers" (original Bezae Cantabrigiensis) (13 words not counted in the totals)
Heb 11:37 "they were tried/tempted" vs. "they were sawn in two" vs. "they were tried/tempted, they were sawn in two" vs. "they were sawn in two, they were tried/tempted" (2 words)
Heb 12:1 "easily surrounding/entangling sin" vs. "easily distracting sin"
Heb 12:3 "hostility against himself for sinners" vs. "hostility from sinners against himself"
Heb 12:18 "touched [something]" vs. " touched [the] mountain"
Heb 13:15 "of/by Him" vs. "through it" (2 words)
Heb 13:21 "every good" vs. "work good" vs. "every work good"
Heb 13:21 "will" vs. "will of him"
Heb 13:21 "of/among us" vs. "of/among you"
Heb 13:21 to the ages "of the ages". vs. "to the ages" (2 words)
Heb 13:25 add "amen" at the end
Some manuscripts add at the end "To the Hebrews written from Italy, by Timothy" (8 words not counted in the totals)
Bibliography for this question: The Greek New Testament Third Edition by Kurt Aland et al., The Greek New Testament Fourth Edition by Kurt Aland et al., Interlinear Greek-English New Testament by George Ricker Berry, the Interlinear Bible by Jay P. Green, The Expositor’s Bible Commentary volume 8, A Textual Commentary on the Greek New Testament 2nd edition by Bruce M. Metzger, The Complete Text of the Earliest New Testament Manuscripts edited by Philip W. Comfort and David P. Barrett, The Expositor’s Greek Testament edited by W. Robertson Nicoll, and footnotes in the NASB, NIV, NKJV, and NRSV Bible translations. Manuscripts of the Greek Bible : An Introduction to Paleography by Bruce M. Metzger also has interesting information on the characteristics and quality of the copying of each manuscript.