This is from the Skeat & Milne book p. 37-38.

Punctuation at the beginning of Matthew is shown to be from the original scribe A.

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The corrections denoted B by Tischendorf and Lake all occur in the first few pages of Matthew. As Tischendorf had himself seen, they form part of an ambitious scheme of correction, annotation, and general improvement of the text, which we have already attributed to scribe A himself (p. 37 above). We have there shown how A soon tired of his task, and within the space of a page or two successively discarded the marking of Old Testament quotations, accents and breathings, paragraphi, and the Eusebian apparatus. The true ‘B’ corrections cease somewhere about the same point, but in a modified form the same script recurs sporadically, and gradually merges into A’s more usual type of correcting hand (Lake’s A3). p. 45
Keep in mind that nobody has actually contested this analysis of Skeat, so all sorts of unusual elements exist in the supposed 4th century Matthew, in the early chapters, written by Scribe A.

Also the important cross-referencing noted by Skeat:

"quotations from the Old Testament marked with arrow-heads (and, in the earlier cases, the name of the book as well)"

Seems to be simply ignored in Sinaiticus literature.

To be fair Jongkind did offer a bit of counterpoint to the Scribe A analysis, this is how it was reported by Charles Evan Hill:

Irenaeus, the Scribes, and the Scriptures. Papyrological and Theological Observations from P.Oxy. 405 (pre-pub. version)
Charles Evan Hill

... Dirk Jongkind, Scribal Habits of Codex Sinaiticus, Texts and Studies, third series, vol. 5 (Piscataway, NJ: Gorgias Press, 2007), 47, note 54, states that he is not convinced that these corrections (called the B corrections) are by scribe A or D. citing the fact ‘‘that the kai-ligature of the B corrections ... is different from both scribe A and scribe D. See e.g. folio 73.3 (NT 3), line 2.12, and 4.19 in the outer margin”. Tischendorf, he notes, thought that these "corrections” came from a slightly later hand not from the original scriptorium. - p. 9
However, this does not really answer the Skeat analysis, which was a correction of Tischendorf. Plus, a "slightly later hand" does not change the anachronism problems.

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2:12 and 4:19 ( a puzzler) the folio count must be the non-CSP one, so we are in folio 74

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Note the super-small print as well. Tiny print.

As for this being a major argument, I doubt it. Not any more than my being two scribes if I write Simonides and Tischendorf in one place and Simonides & Tischendorf in another.