Genuine Bostitch replacement part, this item is sold individually.
There are two of these gaskets used on the compressor models shown below.
They are placed in between the AB-A640050 Valve Plate Assembly.
The purpose of the gaskets is to create a seal so air will not leak out.
Over time these will wear and have to be replaced.
This would be a good time to inspect the Valve plate Assembly for wear also.