it's bulkfill co2 not air i'd say and SHOULD NOT BE FILLED WITH UNREGULATED AIR ABOVE 1100PSI-they start to get valve lock around 1250psi anyway
you can get pcp tubes but without much alteration the 78 isn't suitable for air unless it is regulated fed by a paintball bottle or similar
if it's a std length cylinder it will just have the cap which could be a converted original a Gmac one a Porky Yorkie or one of mine
if it is a long tube one then porky yorkie came up with them to be either bulkfill or 4 caplet(the robber copied his idea when he was a member of UKCAF and why he isn't well regarded by most on there)
if the cap looks to be an original steel one with just the foster fitting added them it will likely be a dual bulk or capsule one
if it has a brass inner with a std steel retainer it's probably a gmac half conversion
if it has a brass retainer and inner a full gmac one
if it has a brass retainer with aluminium inner(some i anodisedblack) and two orings it could be one i made