Mormon simply refers to "these plates" (verse 4) and contrasts them to the better known set, the "plates of Nephi." Mormon is consistent in this reference to what we would call the "large plates" as simply the "plates of Nephi" (Mosiah 1:6, Mosiah 1:16, Mosiah 28:11, Alma 37:2, Alma 44:24, 3 Ne. 5:10, 3 Ne. 26:11, 3 NE. 26:6, 4 Ne. 1:19, and Morm. 1:4). There are two specific verses that are worth particular examination:
Morm. 2:17
17 And now, the city of Jashon was near the land where Ammaron had deposited the records unto the Lord, that they might not be destroyed. And behold I had gone according to the word of Ammaron, and taken the plates of Nephi, and did make a record according to the words of Ammaron.
Morm. 2:18
18 And upon the plates of Nephi I did make a full account of all the wickedness and abominations; but upon these plates I did forbear to make a full account of their wickedness and abominations, for behold, a continual scene of wickedness and abominations has been before mine eyes ever since I have been sufficient to behold the ways of man.
From verse 18 we learn that Mormon (rather like the first Nephi) has written two records. The first is written directly on the "plates of Nephi," and the second is written on "these plates," referring to his abridgment.
Morm. 6:6
6 And it came to pass that when we had gathered in all our people in one to the land of Cumorah, behold I, Mormon, began to be old; and knowing it to be the last struggle of my people, and having been commanded of the Lord that I should not suffer the records which had been handed down by our fathers, which were sacred, to fall into the hands of the Lamanites, (for the Lamanites would destroy them) therefore I made this record out of the plates of Nephi, and hid up in the hill Cumorah all the records which had been entrusted to me by the hand of the Lord, save it were these few plates which I gave unto my son Moroni.
In this verse Mormon's very clearly indicates that the source material for his abridgment was the "plates of Nephi." We are now quite certain that the "plates of Nephi" began as the first set of plates create by Nephi, and they continued to be called the "plates of Nephi" throughout all of the subsequent writers on those plates.
In spite of the use of the plates of Nephi as the primary source material, we must understand that the plates of Nephi were not the only available sources. At the very least we have the brass plates and the 24 plates that contained the record of Ether (Mosiah 8:9). To these we may also add the set of plates we call the "small plates of Nephi." There were yet been more:
3 Ne. 5:9
9 But behold there are records which do contain all the proceedings of this people; and a shorter but true account was given by Nephi.
3 NE. 5:10
10 Therefore I have made my record of these things according to the record of Nephi, which was engraven on the plates which were called the plates of Nephi.
3 NE. 5:11
11 And behold, I do make the record on plates which I have made with mine own hands.
When Mormon refers to "a shorter but true account was given by Nephi" he does so in contrast to "records which do contain all the proceedings of this people." Mormon indicates that rather than use this more exhaustive source, he is abridging the shorter version found in the book of Nephi in the plates of Nephi. This "book of Nephi" refers to the Nephi contemporary with the visit of Christ. What is clearly significant is that there is a set of more complete information which are not on the "plates of Nephi." Therefore, Mormon had access to multiple records, only one series of which was termed the "plates of Nephi."
As a last note, Mormon indicates in 3 Nephi 5:11 that he is writing on plates "which I have made with mine own hands." This is significant because it reminds us that the production of plates was an essential forerunner to the ability to write on them. Were one to run out of space on the inherited set of plates, new ones would have to be made. We have seen that the small plates ended partially because they were full, and there was no room left on which to write on them (Omni 1:30). It is unlikely that Amaleki would have had the resources to make more plates.
In the context of Mormon's records, a single continuously "bound" set of plates cannot describe the "plates of Nephi," particularly when we remember that Mormon had to go looking among his plates after abridging the records up to the time of Benjamin (Words of Mormon 1:3). The physical arrangement of the "plates of Nephi" had to be separate sets of plates for various writers, probably akin to the small plates, where available space allowed some continuation from writer to writer on the same set of plates, but that other writers would have begun on a different physical set. What is important to remember is that this multiplicity of physical plates, each bearing the individual name of the particular author, was yet collectively termed the "plates of Nephi." It would therefore appear that this term, "plates of Nephi," had become a generic label for a particular type of historical record kept by the Nephites, although it was not the only record kept.