Motorcraft 2100, 2150, 4100 carburetor choke thermostat assembly. Included is a picture of the 2100 choke housing you should have for this to work. There is also a version of choke housing that doesn't use a choke piston. That will work also. The one application that doesn't work is the version that has a spring attached to the inside of the choke housing. The center post where the spring attaches sticks out too far not allowing the thermostat to mount.
New, currently being produced.
Includes an aluminum expansion plug for plugging off the hot air.
This is a retrofit for the 4100 carburetor. Same holds true about the housing. Some have the spring loaded mounting post sticking out too far not allowing the thermostat to fit.