1 Nephi 9

As Nephi explains his current record, he tells us that he had written a more lengthy historical record of his people, and now  -

"...I have received a commandment of the Lord that I should make these plates, for the special purpose that there should be an account engraven of the ministry of my people."  (Book of Mormon | 1 Nephi 9:3)

Adding -

"...the Lord hath commanded me to make these plates for a wise purpose in him, which purpose I know not."  (Book of Mormon | 1 Nephi 9:5)

After Martin Harris lost the first 116 pages of the translated record, the Lord instructed the Prophet Joseph Smith -

"...you shall translate the engravings which are on the plates of Nephi, down even till you come to the reign of king Benjamin, or until you come to that which you have translated, which you have retained..."  (Doctrine and Covenants | Section 10:41)

Also adding -

"Behold, there are many things engraven upon the plates of Nephi which do throw greater views upon my gospel; therefore, it is wisdom in me that you should translate this first part of the engravings of Nephi, and send forth in this work."  (Doctrine and Covenants | Section 10:45)

Just a couple of thoughts.  First, we learn that Joseph smith had retained some of the pages of the original translation.  Second, he was instructed to translate the small plates of Nephi down to the reign of kin Benjamin.  I have no idea if there was more on the small plates of Nephi or if Amaleki was the last to write on these plates.

Mormon explains after he had abridged the record of Nephi up to king Benjamin, that he came upon these plates and decided to include them in his abridged record.  He even added an explanation and a little history to tie this record to the last days of king Benjamin.  I have often wondered if Mormon actually wrote them again on his gold plates or somehow inserted them into his abridged record.