Viton choke plunger gasket pair. Fits into the choke plunger (13937800) of VHB carbs. Plunger not included but also available. 

One end is slightly smaller than the other. Start on the small end to get the gasket into the plunger. There is a very slight lip that the surface needs to go under to flatten the gasket correctly. 

Sold as a pair.

These are made by Bruce Giller. He has a detailed write up on thisoldtractor regarding gaskets for the wide variety of petcocks on loop and Tonti Guzzis.