What a gorgeous thing! I'd never seen a stencil wheel like this before, and think it is just fantastic; I certainly love interlocking brass letter and number stencils, but they don't hold a candle to this, where one has all the letters and numbers together, plus a few handy extras (lbs, yds, $, etc.) Held together at the center with a turned wood handle, the arm on top rotates easily around the disc, with two windows for framing the letter or number to the be used.
While the patent information of this has become difficult to read, I can read "Aug 11" and have found a couple other examples of this wheel out there that show patent dates of Aug. 11, 1868 and May 30, 1871, which I assume is the case here. Others I have found also have a mark for the distributor (i.e. New York Stencil Works, NYC or Allen Bros. Boston) but I can't see the equivalent on this one.
The aging and greening to the brass really enhances its beauty as an object, an it would be just stunning on the wall, ideally hung such that the wheel can still be turned to spell out messages as needed! (This would definitely make prepping for a spelling bee more fun!) 14 1/8" across at arm; the disk itself measures 13" across. There is a little bit of bend to the arm, as pictured.