Plastic choke thermostat cover. Use as a universal type of thermostat on some carburetors. The hot air tube would connect to the side of the thermostat housing.
Diameter = 2.25" (2 1/4")
Add a universal thermostat spring with will work clockwise, or counter-clockwise.
Use one of these springs:
The difference is the # of coils. Try to match your old thermostat as close as you can.