I saw Crazydoug's post on his tiny single cylinder TMY and had to send some photos here of mine, which is quite different (although also exactly 2 inches tall). I have been unable to find any references to this engine whatsoever, but I am familiar with the other models he noted, and I have both the smaller and larger versions of the flat-twin model (last photo shows that engine with box next to this tiny single cylinder).
I am not sure exactly what sort of valve mechanism this has, but it runs like a top, in either direction depending on which intake pipe is used. The cylinder cap has a small port on top, and unscrews to reveal the piston. Oddly (?), it runs just as well with the cap removed! Have only run it on air, but I would guess that it puts out an impressive stream of steam exhaust from that port. The cylinder bore is about 7.5 mmm (5/16 inch).
Anyone ever seen another like this?