These are OEM everseal R1, R2 caps.  We were able to scoop up a few in an auction for our purist customers.  When they're gone they are gone.  

This is the conventional size and fits a variety of other vehicles that  utilize the inverted cup that rides and "formed ramp" until it strikes the downward facing tab stop.  Its commonly called a bayonet-style cap and was used with with non pressure systems. A neck is included for illustration purposes e.g. you are buying a cap and not buying a neck.

We have tested these and we estimate that the fiberboard seal and inverted shallow brass cup behind it create a resistance of ~3psi.  It is intended for non pressure systems