I love this thing. The wonderful dealer I purchased it from (an artist who loves curious little things like this as much as I do) thought it along the lines of a cracker-jack-box ciphering wheel/decoder, and I think that's right, creating a correspondence between numbers and letters, which one could use for writing, or deciphering, messages written in secret code. The center dial turns, and is incised with lines, as is the outer disk. So one course, say, align "A" with "1" and go from there--i.e. M=2, Z=3, etc. Made of lead I believe, with a nice weight for its size, and just really cool!

2 5/8" in diameter. Good condition, smaller disk is held tight but turns readily. Disks incised with dial lines  and numbers and letters; the numbers and letters gone over with ink.