501 Front assembly, without the release valve, spring, seal,  and Retaining Ring, but includes the front end Dome cap cover Front body and O ring. Does not use the relief valve.

Reduces the risk of gas leak from the relief Valve.

Before you remove the front when the gas has expired make sure that there is no pressure left in the chamber or the pressure (even low) will try to force it out into the distance.

Photo shows the standard one below for comparison.
Photo shows the Stainless Steel front cap Front body and front ring (diamond knurled) fitted and the left shows the standard plastic one. This price is a bargain.  Now also named Rabbit Destroyer.
The last photo shows my front in stainless Steel with the relief valve if required which is also on this site.