Verse 3 begins with the phrase “and these plates of brass”, which is modified by a sequence of three relative clauses that all begin with which. Then, without apparently providing any predicate for the initial noun phrase, the text abruptly shifts and starts over in verse 4 with a new clause that nonetheless refers to “these plates of brass”—namely, “and behold it hath been prophesied by our fathers that they [the plates of brass] should be kept and handed down from one generation to another”. The and that begins verse 4 heightens the sense of abruptness, which may explain why the editors for the 1920 LDS edition deleted it. They also decided to replace the original period at the end of verse 3 (identified above at position 2) with a dash, thus providing a more direct link between verses 3 and 4. The removal of the and was intentional since it was marked in the 1920 committee copy.
But as David Calabro points out (personal communication), the real problem here is that the 1830 typesetter placed a semicolon between verses 2 and 3 (that is, at position 1); the 1920 LDS edition replaced the semicolon with a period, but this did not affect the sentential structure. Calabro argues that the phrase at the beginning of verse 3, “and these plates of brass”, actually serves as part of the direct object for the earlier verb keep in verse 2. By omitting the parenthetical statements in verse 2, the relationship becomes clear:
In other words, Alma commands Helaman to keep all these things sacred, not only the Nephite records and artifacts but also the plates of brass. Under this interpretation, the sentence beginning verse 2 is completed at the end of verse 3. Consequently, verse 4 begins a new sentence (“and behold it hath been prophesied by our fathers that …”), which makes perfectly good sense. Thus there was no need in the 1920 edition to remove the and at the beginning of verse 4 and to replace the period at the end of verse 3 with a dash.
Summary: For Alma 37:2–3, the punctuation at the boundary between verses 2 and 3 should be emended so that these two verses act as a single sentence; correspondingly, the original full stop, a period, at the end of verse 3 should be restored since verse 4 begins a new sentence; consequently, the original and at the beginning of verse 4 should also be restored.