Here Oliver Cowdery omitted the modifier most in his copying from 𝓞 to 𝓟. We have already seen one other place where he made the same error:
Close by the example in 1 Nephi 13:28 is another one involving most, but this time the problem occurs in 𝓞 rather than 𝓟:
In this instance, scribe 2 of 𝓞 first started to write precious after the and, but then he caught his error, erased the pre that he had already written, and overwrote the erased text with the word most. Finally, he crossed out this second most. Perhaps he consciously deleted the most because of its apparent redundancy with the most in front of plain, in which case the original text actually read “most plain and most precious”. On the other hand, perhaps he accidentally repeated the most and for that reason he deleted it, in which case the original text read “most plain and precious”.
Besides these examples, there are ten occurrences elsewhere in the text of precious preceded by most, of which the following five are found conjoined with another adjective:
Note in particular the example in 1 Nephi 15:36 where the most is repeated. This example shows that there would be nothing wrong with “the most plain and most precious parts” in 1 Nephi 13:34. Since scribe 2 of 𝓞 did not edit the example in 1 Nephi 15:36 of the repeated most, the deletion of the repeated most in 1 Nephi 13:34 was probably an attempt to make the written text agree with what Joseph Smith had dictated, not what scribe 2 thought sounded better.
In support of this argument, consider examples where a conjoined precious is not immediately modified by most:
The example in 1 Nephi 19:3 (“the more plain and precious parts”) shows that the corrected reading in 1 Nephi 13:34 (“the most plain and precious parts”) is possible.
David Calabro points out (personal communication) that scribe 2 of 𝓞 might have accidentally crossed out the wrong most in 1 Nephi 13:34; after correctly inserting the most before precious, perhaps scribe 2 should have deleted the earlier most (the one before plain), but in his confusion he accidentally deleted the one he had just inserted. If so, the original text in verse 34 would be “the plain and most precious parts of the gospel of the Lamb which hath been kept back by that abominable church”, which would have been completely identical with the phraseology two verses before, in 1 Nephi 13:32 (“the plain and most precious parts of the gospel of the Lamb which hath been kept back by that abominable church”).
Obviously, a number of conjectures are possible. Perhaps the best solution in 1 Nephi 13:34 is to accept scribe 2’s final corrected text (without the repeated most) since this reading will work. Scribe 2 of 𝓞 had just written “the plain and most precious parts” two verses earlier (in 1 Nephi 13:32) and ended up accidentally writing “the most plain and most precious parts” in verse 34. He caught his error and deleted the second most.
Summary: Restore the most in “many plain and most precious things” (1 Nephi 13:28); Oliver Cowdery accidentally deleted this most when he copied the text from 𝓞 into 𝓟; in 1 Nephi 13:34, scribe 2’s final reading in the original manuscript (“the most plain and precious parts”) is probably the original text.