I think the best fittings one is for air.
Just been out to the garage to check on mine for a C25 and it is nothing like, two female threads.
I see by a search on hear Wonky Donkey had some some years ago, might be worth a pm.
Sorry I read that wrong, strike out Wonky Donky, as far as I know all FWB CO2 Adaptors are the same.