Isaiah 6:5 (King James Bible) for I am undone because I am a man of unclean lips
Here in 2 Nephi 16:5, the original text (and the original manuscript) undoubtedly read like the corresponding Isaiah passage except that the italicized am of the King James text was omitted. When Oliver Cowdery copied from 𝓞 to 𝓟, he accidentally skipped “I am undone because”. Later, probably when proofing 𝓟 against 𝓞, he supplied the missing text supralinearly (and with somewhat heavier ink flow). The probable reason for the error is that Oliver’s eye skipped from the I am of “I am undone” to the I a m of “I a man”.
The italicized King James am in “I am a man” was intentionally omitted from the original Book of Mormon text. On the other hand, the nonitalicized am in the preceding “I am undone” was in the original text. In the Hebrew, the equivalent for “I am undone” is a single verb form with the meaning ‘I am destroyed’; in other words, the verb is not a case involving the linking verb be (as is “I am a man of unclean lips”). In his editing for the 1837 edition, Joseph Smith supralinearly supplied the am for “I am a man of unclean lips”. The critical text will, of course, restore the original text without that am (“I a man of unclean lips”) but will maintain the am of the preceding “I am undone”.
Summary: The original text in 2 Nephi 16:5 had the am in “I am undone” but omitted it in “I a man of unclean lips”, which means that the original Book of Mormon text follows the corresponding literal Hebrew in Isaiah 6:5.