I've been trying to find out the history and names of the 2 (technically 3 if you're counting left over Mk.2 No.1 S6 filters) filters used with the S10 and exactly what designations were used, the later more common PS10 filter's name i do know however its designation might've been the L12A1? been trying to confirm this via freedom of information requests to the MoD but its akin to pulling teeth, there is also the other earlier filter which was used in the S10's trials which i know not the name nor history behind. If anyone could help with general information and designations of these it'd be a massive help. I've attached photos of the earlier filter if it is of any help.