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The manuscripts that include 'ton prwtotokon' are: C, D, L, W, 087, Majority Text, aur, f, ff¹ , vg, sy^p.h

-A8

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See this note on how it got in there:
1:25. uion. TR adds authV ton prwtotokon "her firstborn" after uion, according to C D L W, most cursives, some Old Latin MSS, the Vulgate, and the Peshitta Syriac. We omit it with Lach. Tisch. Treg. Alf. West. Nest. UBS., according to S B Z and some cursives, some Old Latin MSS, the Old Syriac, and the Coptic versions. Probably scribes inserted the words from the parallel in Luke 2:7.

If you check Luke 2:7, even Westcott and Hort include ton prwtotokon in their translation.