I've found a couple of these wonderful old brass stencil wheels over time, and keep an eye out for them as I think they're just fantastic things. This early one, with an 1871 patent, is just about the best I've seen, still in excellent condition but with a gorgeous patina, and completely useable should one want to put it to work again. There were a few different sizes and variations of these made; this is the largest I believe, including all the letters and numbers as well as handy extras (lbs, yds, $, etc.) Held together at the center with a wood handle, the arm on top rotates easily around the disc, with two windows for framing the letter or number to the be used.
In addition to the patent mark, there is a mark for Allen Bros. Boston, who would have been the distributor.
14 1/8" across at arm; the disk itself measures 13" across. Really excellent antique condition, with next to know bending of the brass at all. Perfect hung on the wall from the handle to or from a letter with the handle hanging down.