V87403L Cylinder Gasket for Halkey Robert Alpha Inflator also known as the V90000.
The V87403L Cylinder Gasket can be bought individually or in a Multi Pack which you can find on this link.
This gasket is the 1/2″ size item.
The cylinder gasket helps guard against the cylinder coming loose. After tightening, the gasket creates a reverse pressure against the thread. It can also help alleviate any gas leakage during inflation. Cylinders should be checked regularly to make sure they are screwed tight.
Click here for information on the Alpha Inflator.