I cringe when I hear about people hooking up grenade cans. The system could have anything, and I mean anything in it. Adding R134 without hooking up gauges and also finding out what's in there will cause black death to the a/c system.

The system could have any number of drop in gas(Freeze-12, ect), R134a, R12, R22, propane, butane.

If you don't know what you are doing you could hurt yourself, bad.