http://en.wikipedia.org/wiki/1,1,1,2-Tetrafluoroethane
it may not be marked 134a, it may be listed by the chemical name in the contents.
It's an automotive ac refrigerant that was replaced, so it's somewhat regulated because it's ozone depleting when released to atmosphere... it's also technically against the law to release it to atmosphere lol.
The important thing here is that it's increasingly harder to get and the chemicals that replaced it are not so friendly on regular seals and rubber. So if you use the unmarked stuff and suddenly develop all sorts of leaks, there's or your reason.