This is a sleeve throttle. It fits certain styles of Cox cylinders. The cylinder fits in side this sleeve and is then screwed back into the crankcase. You position the two exhaust ports inline with the ports on the cylinder. The snap ring can be repositioned anywhere in a 360 degree area, which will have a wire pushrod hooked to it.

The way it works is when the ports in the sleeve and the cylinder are lined up, you have full throttle. By closing off the cylinder ports, by rotating the sleeve, you restrict the exhaust, thus providing some throttle control. It doesn't work as well as a carburator, but it does work.
It's a simple, basic exhaust throttle.

These are hard to find.