Yes JD, it is the bottle/tank that is tested - you are quite correct there, but it does in fact matter where the bottle is to be used mate.
Diver's tanks that are used for filling of air rifles will have a valving system that can only be used on land (also termed 'surface use only') and not under water, and as such, are only subject to a 5 yearly hydrostatic test.
Diver's tanks that are to used for the breathing of either compressed air, or a mixture of gasses thereof for the same use under water have a totally different valving system to that used on tanks for air rifle filling (wat