In the second half of verse 17, the original manuscript reads his promise, although earlier in the verse we have the plural all his promises. The preceding plural usage probably influenced the 1837 printer (or perhaps Joseph Smith) to change the following singular his promise to his promises. Nonetheless, we have two different kinds of promises in this passage. In verse 17, Alma is telling Helaman that God will fulfill all his future promises (“all his promises which he shall make unto you”), just as he has already fulfilled a previous promise made “unto our fathers”—namely, that the records would be preserved (“that he would preserve these things for a wise purpose in him”). We can find specific reference in the small plates of Nephi to this particular prophetic promise made centuries earlier to Lehi and to Nephi:
Thus the occurrence of the singular promise in the latter half of Alma 37:17 should be restored to the text.
We also note here a minor variant at the end of verse 17: namely, in the original manuscript, Oliver Cowdery initially wrote the singular fother (a scribal slip for father). Shortly thereafter Oliver corrected this singular to the plural by inserting inline the plural s (there is no change in the level of ink flow for the s). This correction undoubtedly reflects the reading of the original text. Note, for instance, that the following plural them supports fathers: “for he promised unto them that he would preserve these things for a wise purpose in him”. And from the two quotations from the small plates of Nephi (cited above), the small plates record this promise as having been made to both Lehi and Nephi. Moreover, the use of a singular our father here in Alma 37:17 seems virtually impossible unless the name of that father was added, as in Alma 9:9 (“our father Lehi”), Alma 13:15 (“our father Abraham”), or 3 Nephi 10:17 (“our father Jacob”). Finally, we have explicit evidence later in this chapter of Oliver’s tendency to initially write father rather than the correct fathers. In that instance, the initial error was in 𝓟 but was corrected by reference to 𝓞, which is extant and reads twice as Fathers:
The plural fathers is undoubtedly the correct reading in Alma 37:17 as well.
Summary: In accord with the reading of the earliest textual sources (𝓞, 𝓟, and the 1830 edition), restore the singular his promise in the second half of Alma 37:17; the preceding all his promises refers to future promises, not the particular one made centuries earlier to Lehi and to Nephi that the plates would be preserved; contextually, the use of the plural our fathers is correct in this passage.