I have serviced millions of Airsporters ... no really... the metal ring you refer to was BSA`s ide of a compression ring. Perhaps they thought adding it would make it like a FWB 300s..!!!!
It reduces available compreesion by shortstoking the piston and adds considerable friction to boot.! A total waste of effort BSA .
Bin it and fit an earlier or later standard O ring head.

Pete.