The original manuscript has the third person plural verb form contain rather than the third person singular verb form contains or containeth. It is quite possible that this usage is due to the immediately preceding plural noun Jews, which is not the grammatical antecedent of the which
but whose immediate proximity leads to the choice of the plural verb form contain. The following example shows another case where an immediately preceding plural noun (the wars) determines the verb form, even if the actual grammatical antecedent (the record) is singular:
In other words, the immediate proximity of the plural wars leads to the plural verb form are because “wars are” is expected, not “wars is”. For additional examples in the original text of the influence of an intervening prepositional phrase on grammatical number, see subject-verb agreement in volume 3.
Another possible explanation for 1 Nephi 13:23 is that scribe 2 of 𝓞 may have accidentally dropped the s from contains. Dropping the final s is a common scribal error for scribe 2, as these examples later on in 1 Nephi 13 show:
All of these examples, however, involve the plural s and not the third person singular s.
When Oliver Cowdery copied the verb contain into 𝓟, he added the s, although another possibility would have been to add -eth. Support for this grammatical emendation comes from the immediately following sentence in this verse:
On the other hand, contains does occur in the Book of Mormon:
On balance, all of this evidence suggests that the use of the plural contain in 1 Nephi 13:23 was the result of the immediately preceding plural noun Jews. The use of containeth later on in the verse argues that the third person singular form contains was not intended—and that containeth might have been a more systematic grammatical emendation for Oliver Cowdery to have made.
The critical text, consistent with the original (but nonstandard) usage in Jarom 1:14 (which is based on proximity), will restore the nonstandard usage in 1 Nephi 13:23.
Summary: Restore the ungrammatical “the book … is a record of the Jews which contain the covenants of the Lord” since usage elsewhere suggests that nonstandard subject-verb agreement does occur in the earliest text.