Sunday School lesson from 1936 teaches that the Urim and Thummim helped with Book of Mormon translation.
Sunday School manuals; Sunday School Lessons Volume 9, 1936; Church History Library, 18-22 (146)
Joseph arrived in Harmony in December. He started to work right away in copying some of the strange characters from the plates. With the use of the Urim and Thummim he was able to translate some of these...
[I]t was so slow for one man to read the plates through the Urim and Thummim and then stop to write it down.