Mosiah 28:20 says the following:
"after king Mosiah had done these things, he took the plates of brass, and all the things which he had kept, and conferred them upon Alma, who was the son of Alma; yea, all the records, and also the interpreters, and conferred them upon him, and commanded him that he should keep and preserve them, and also keep a record of the people, handing them down from one generation to another, even as they had been handed down from the time that Lehi left Jerusalem."
Mosiah 28:20 uses the phrase "they had been handed down from the time that Lehi left Jerusalem." Does this phrase refer to "all the things that he had kept”? In other words, were "all the records, and also the interpreters" "handed down from the time that Lehi left Jerusalem"? Or are the interpreters not meant to be included in that reference? If the interpreters are part of that reference, then they were handed down from generation to generation either by the Nephites or the Mulekites.
According to Bruce R. McConkie, there is no record that Lehi brought a Urim and Thummim to this continent, but King Mosiah had one prior to the discovery of the Book of Ether, and it was handed down from prophet to prophet. (Omni 1:20-21; Mosiah 8:13-19; Mosiah 21:26-28; Mosiah 28:11-20; Alma 63:12; Ether 4:1-7). [Bruce R. McConkie, Mormon Doctrine, p. 818]
According to Verla Birrell, the Book of Mormon does not explain how Mosiah2 received these stones. Were they given to the people of Zarahemla by Coriantumr, and then to the Nephites by the people of Zarahemla? Were they found with the stone tablet of Coriantumr? Mosiah2 apparently was in possession of them before the plates of Ether were found and brought back to him by Limhi (Mosiah 8:13-19) [Verla Birrell, The Book of Mormon Guide Book, pp. 264-265]
The first edition (1830) of the Book of Mormon names Benjamin as the king of Zarahemla who possessed the interpreters (see p. 200). If so, how did Benjamin get them?
One other option is that the interpreters were a gift of God to either King Benjamin or King Mosiah2. In Mosiah 8:13, Ammon said, "I can assuredly tell thee . . . of a man that can translate the records; for he has wherewith that he can look, and translate all the records that are of ancient date; and it is a gift from God. And the things are called interpreters" (my italics). If all the italicized phrases refer to the interpreters, then we can say that either King Benjamin or King Mosiah2 received the interpreters as a gift from God, whether by angelic messenger or from Ether himself. [Alan C. Miner, Personal Notes]
According to Bruce R. McConkie, Joseph Smith received the same Urim and Thummim had by the Brother of Jared for it was the one expressly provided for the translation of the Jaredite and Nephite records. (D. & C. 10:1; 17:1; Ether 3:22-28) It was separate and distinct from the one had by Abraham and the one had by the priests in Israel. [Bruce R. McConkie, Mormon Doctrine, p. 818] [See the commentary on Omni 1:20; see also the extended commentary on Ether 3:22-28] [See 1 Nephi 5:4]