The idea of a heavenly being showing a mortal "all the inhabitants of the earth" (Ether 3:25) is not without precedent. Midrash Rabbah Exodus 40:2 notes that as Moses was about to descend from the mountain, God "brought him the book of Adam and showed him all generations that would arise from Creation to Resurrection, each generation and its kings, its leaders, and prophets."
Terrance Szink, of the Department of Ancient Scripture at Brigham Young University, suggests that Moses also received the urim and thummim with the records on Mount Sinai. The sacred oracle, first mentioned in Exodus 28:30 appears in such a way as to hint that it already existed before the high priestly clothing with which it became associated was made.
Szink also suggests that the tables of stone given to Moses on the mountain may not have been prepared by God at that time but may have been hidden there earlier, under the Lord's direction, by a prophet charged with protecting the ancient records. The fact that the two tables had been "written with the finger of God" (see Exodus 31:18: 32:15-16; compare Deuteronomy 4:13; 5:22; 9:9-11) does not mean that he wrote them at the time that he spoke with Moses on the mount. After Moses broke the two tables (see Exodus 323:19; Deuteronomy 9:15-17), the Lord did not simply write up another set but had Moses prepare the new tables (see Exodus 34:1,4). Moses then spent forty days with the Lord, during which time the prophet, not the Lord, wrote on the tables (see Exodus 34:28-29). [John Tvedtnes, The Book of Mormon and Other Hidden Books: Out of Darkness unto Light, pp. 129-131] [See the commentary on 1 Nephi 14:26; 2 Nephi 27:7]