These three verses close the section concerning the translation of the records from the people of Limhi. As a conclusion they are quite disappointing.
Verse 17 is the only information given at this point that describes the contents of that record. While Mormon clearly thinks that the content was important, because he will include it later (verse 19: these plates serve as the text for the Book of Ether added much later) at this point he gives only the tersest of synopses.
When Moroni does write about the contents of these records we learn that there was a section of the translation of these plates that was commanded to be withheld until after Christ had been born (Ether 4:5-6). Mormon apparently honored that wish chronologically, even though Mormon was clearly writing after Christ's birth.
What is also missing from our current edition of the Book of Mormon is the complete finality of these short verses. These verses end a chapter in the 1830 edition.
Thus for the original reader of the Book of Mormon, there is a great build up to Mosiah as the possessor of these interpreters, and the fact of his translation, and then a very rapid close to the chapter, with only the promise that the text would come later. While it is true that it does come later, it comes only after Mormon has finished with absolutely everything he had to say, and gave the plates over to his son Moroni, who finally includes the text (Ether 1:1). Thus Mormon himself might have been content to leave this text out entirely, even though he makes a big deal of their translation.