By way of explanation, Mormon has been abridging "the Large Plates of Nephi." So far, he has completed the abridgement down to the reign of king Benjamin. This record has been lost to the world. The translation was contained on the 116 pages that Martin Harris lost. Mormon, while abridging this record, must have read about a reference to the small plates (see DC 10:39). This prompted him to take a break in abridging the Large Plates to search for another record. This other record, called these plates, are "the Small Plates of Nephi." The small plates cover the same time period and have a spiritual tone which will later provide an able substitute for the record which was lost.
It is important to understand that this chapter was written after Mormon had abridged the record of Lehi, but before he had abridged Mosiah through Mormon. The evidence for this is as follows, 1) Mormon plainly states that he found the small plates after he had made an abridgement of the plates of Nephi (large plates) down to the reign of this king Benjamin. He had not yet finished abridging the entire record of the large plates, 2) He states in verse 5 that the remainder of his record (Mosiah through Mormon) will be taken from the plates of Nephi (as yet unabridged large plates), and 3) In verse 9, Mormon says, And now I, Mormon, proceed to finish out my record, which I take from the plates of Nephi. Apparently, Mormon is just about to begin his abridgment of the rest of the record. One could imagine Mormon in a room or cave filled with plates. He has been working hard on engraving the Plates of Mormon (see "A Brief Explanation About the Book of Mormon" found in the front of the Book of Mormon). He has completed the abridgment from Lehi to Benjamin when he stops to find the small plates. He includes the small plates with his plates, writes the insert known as the Words of Mormon, and then continues on to abridge the huge history which lay before him in the Large Plates of Nephi. No wonder he was left to exclaim, I cannot write the hundredth part of the things of my people (v. 5).
We learn from these verses that Mormon's work on the writings contained in the Book of Mormon took place just prior to the battle of 385 AD. This places his age over 70 and demonstrates his perspective as he abridges the record. Mormon is writing the words we know as the Book of Mormon after collecting all the wisdom of age and experience. He commonly includes stories of the Gadiantons because he has already seen the role the Gadiantons played in the final destruction of his people (Mormon 1:18). He is fatalistic about the Nephites, not just because he knows of prophecies concerning their destruction, but because he has already witnessed their fall. We found the same pattern in the life of Nephi, who began writing the record we know as 1 and 2 Nephi long after many events had already transpired—as late as 569 BC (2 Nephi 5:28-30).