Acts 2:26

[John Reece]

I.... This understanding would explain the late addition of του to the text to read Αβιάθαρ του αρχιερέως.

The "late addition" of του?

What is the basis for your assumption that the text of Nestle-Aland (ἐπὶ Ἀβιαθὰρ ἀρχιερέως) is late and that of Textus Receptus (ἐπὶ Ἀβιαθὰρ τοῦ ἀρχιερέως) is early?

From The King James Version Debate: A Plea for Realism (Baker, 1979) by D. A.Carson:

There is no unambiguous evidence that the Byzantine text-type [which includes the reading in Textus Receptus] was known before the middle of the fourth century This point may be established by: (1) determining if there are any Greek manuscripts of pre-A.D. 350 date which reflect the Byzantine text-type; (2) examining pre-A.D. 350 versions with the same information; (3) reading the New Testament quotations found in the writings of the pre-A.D. 350 church fathers to discover if the biblical passages they quote approximate any particular text-type. In each case the evidence is uniform: the mature Byzantine text-type appears nowhere before the fourth century. [page 44]

....

The Alexandrian text-type [which includes the reading in Nestle-Aland] has better credentials that any other text-type now available. .... The famous papyrus p⁷⁵, which dates from about A.D. 200 and is perhaps earlier, is astonishingly close to Vaticanus. This find definitely proves the early date of the Vaticanus text-type. [page 55]

[37818]

The "late addition" of του?

What is the basis for your assumption that the text of Nestle-Aland (ἐπὶ Ἀβιαθὰρ ἀρχιερέως) is late and that of Textus Receptus (ἐπὶ Ἀβιαθὰρ τοῦ ἀρχιερέως) is early?

From The King James Version Debate: A Plea for Realism (Baker, 1979) by D. A.Carson:

There is no unambiguous evidence that the Byzantine text-type [which includes the reading in Textus Receptus] was known before the middle of the fourth century This point may be established by: (1) determining if there are any Greek manuscripts of pre-A.D. 350 date which reflect the Byzantine text-type; (2) examining pre-A.D. 350 versions with the same information; (3) reading the New Testament quotations found in the writings of the pre-A.D. 350 church fathers to discover if the biblical passages they quote approximate any particular text-type. In each case the evidence is uniform: the mature Byzantine text-type appears nowhere before the fourth century. [page 44]

....

The Alexandrian text-type [which includes the reading in Nestle-Aland] has better credentials that any other text-type now available. .... The famous papyrus p⁷⁵, which dates from about A.D. 200 and is perhaps earlier, is astonishingly close to Vaticanus. This find definitely proves the early date of the Vaticanus text-type. [page 55]

Respectfully, you took what I said backwards.

The late reading: επι αβιαθαρ του αρχιερεως
The early reading: επι αβιαθαρ αρχιερεως. And I believe is the original.

The αβιαθαρ αρχιερεως is being translated typically as "of Abiathar the high priest."
It could be understood as "Abiathar of [the] high priest" by the hearers. The hears of Jesus day knowing that event Jesus related regarding Abiathar, that Abiathar was not yet the High Priest but was known to be the son of the High Priest.

So a translation: "Abiathar was High Priest," which is standard, could be rendered "Abiathar was [son of the] High Priest Where, I think, the reader can understand [son of the] is an interpretation which can be omitted. As I explained the hears would have known Abiathar was not yet the High Priest and was the son of the High Priest. So how ever one translates this that common sense understand of the hearers should be considered.

An added note: Both the critical texts and the majority texts (83.7%) have: επι αβιαθαρ αρχιερεως
It is the so-called TR which has: επι αβιαθαρ του αρχιερεως

Also BTW I had not originally checked the reading: επι αβιαθαρ του αρχιερεως. I was aware of επι αβιαθαρ αρχιερεως .reading, but did not know it was the common reading of the Greek texts when I made my OP.

[John Reece]

Respectfully, you took what I said backwards.

The late reading: επι αβιαθαρ του αρχιερεως
The early reading: επι αβιαθαρ αρχιερεως. And I believe is the original.

The αβιαθαρ αρχιερεως is being translated typically as "of Abiathar the high priest."
It could be understood as "Abiathar of [the] high priest" by the hearers. The hears of Jesus day knowing that event Jesus related regarding Abiathar, that Abiathar was not yet the High Priest but was known to be the son of the High Priest.

So a translation: "Abiathar was High Priest," which is standard, could be rendered "Abiathar was [son of the] High Priest Where, I think, the reader can understand [son of the] is an interpretation which can be omitted. As I explained the hears would have known Abiathar was not yet the High Priest and was the son of the High Priest. So how ever one translates this that common sense understand of the hearers should be considered.

An added note: Both the critical texts and the majority texts (83.7%) have: επι αβιαθαρ αρχιερεως
It is the so-called TR which has: επι αβιαθαρ του αρχιερεως

Also BTW I had not originally checked the reading: επι αβιαθαρ του αρχιερεως. I was aware of επι αβιαθαρ αρχιερεως .reading, but did not know it was the common reading of the Greek texts when I made my OP.