The small plates were kept with the large plates from the time of king Benjamin until the days of Mormon. Ammaron was the custodian of the plates in Mormon's day. He saw that Mormon was a sober child and decided to give him custody of the plates, but instructed them not to obtain them until the age of 24 (Mormon 1:2-3). Presumably at age 24, Moroni goes to the specified location, the hill Shim, and takes the Large Plates of Nephi into his possession. He leaves all the other records in the hill. He takes the Large Plates and begins his own story—the unabridged history of Mormon. This is presumably the long version of what we find in Mormon chapters 1-7. It is not until Mormon is 64 years old when he actually takes all the other plates out of the hill Shim (Mormon 4:23). Remember that there were many, many records other than the large and small plates. Mormon kept these plates for 10 years until he hides them up in the hill Cumorah. He places them in the hill right before the final battle so that they won't fall into the hands of the Lamanites (Mormon 6:6).
"The following are the names of the Nephite historians, with the times during which they held the plates. Where a blank space is noted, the information is not given in the Book of Mormon, and therefore can only be guessed at, which we choose not to do.
"Nephi, from _____ to 546 B.C.
Jacob, from 546 to ______.
Enos, from _____ to 422.
Jarom, from 422 to 362.
Omni, from 362 to 318.
Amaron, from 318 to 280.
Chemish, from 280 to _____.
Abinadom, from _____ to _____.
Amaleki, from _____ to 200 (circa).
King Benjamin, from 200 to 125.
King Mosiah, from 125 to 91.
Alma, the Younger, from 91 to 73.
Helaman, the Elder, from 73 to 57.
Shiblon, from 57 to 53.
Helaman, the Younger, from 53 to 39.
Nephi, from 39 to 1.
Nephi, the Disciple, from 1 to A.D. 34.
Nephi, the son of the Disciple, 34 to 110.
Amos, from 110 to 194.
Amos, the Younger, from 194 to 306.
Ammaron, from 306 to 320.
Mormon, from 320 to 385.
Moroni, from 385 to 420. Record closes."
(Reynolds and Sjodahl, Commentary on the Book of Mormon, vol. 2, p. 27-8)