Alma makes note of two sets of records, the plates of Nephi (verse 37) and the plates of brass. Of course there is a conceptual difference between the two in that Helaman may write upon the plates of Nephi, but not upon the plates of brass. One point that should be noted is that Alma gives only two sets of plates rather than the three that we typically consider to be the essential parts of the Nephite record. In our vocabulary, we have the large plates of Nephi, the small plates of Nephi, and the brass plates. Why does Alma only mention two of them?
While we understand the two sets of plates of Nephi with different labels, apparently Nephi used the same name for both:
1 Nephi 9:2
2 And now, as I have spoken concerning these plates, behold they are not the plates upon which I make a full account of the history of my people; for the plates upon which I make a full account of my people I have given the name of Nephi; wherefore, they are called the plates of Nephi, after mine own name; and these plates also are called the plates of Nephi.
It would be simple, therefore, to presume that the reason that we have only a single reference is that the two textual traditions we understand were subsumed under the same name. While the idea that one set was subsumed in the other is possible because of the name, it is more likely that there is a different answer. This more likely answer comes from understanding Mormon’s comment about the small plates of Nephi that he inserted in his record.
Words of Mormon 1:3
3 And now, I speak somewhat concerning that which I have written; for after I had made an abridgment from the plates of Nephi, down to the reign of this king Benjamin, of whom Amaleki spake, I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi.
What this suggests is that there were numerous records, and that the small plates of Nephi were simply among those that were part of Mormon’s source material. However, note that he does not find them until he is writing on the history of Benjamin, the king during whose reign the small plates were reunited with the large plate provenance. This suggests that the small plates may have been included in the records of Benjamin’s reign, and were otherwise no longer separated from the large plate tradition. As we noted in the original discussion of Nephi’s plates, it is quite probable that the physical size of a single sheet of either textual tradition would have been the same, and thus the insertion of what we call the “small plates” (because they contain less material, not because they are physically smaller) would not create any readily noticeable variation in the source materials available to Mormon.
Internal Reference: Verse 5 references an apparent tradition among the Nephites that the brass plates would “…retain their brightness; yea, and they will retain their brightness…” The antecedent that we have to this tradition is found in Nephi:
1 Nephi 5:19
19 Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time. And he prophesied many things concerning his seed.
See the comments following 1 Nephi 5:19 for more information.