You introduced this thread by implying that the old R-12 system was short on gas. Chances are you are going to have to pull the remaining charge to fix it (for smaller systems, anyway, there is no place to pump it down). At that point - you have an empty R-12 system. It doesn't really matter what the interim gas was - use your favorite R-12 alternative to get it running again.
Some other points: If you don't know what it is, that makes disposal a real problem. You can get an EZ One Shot cylinder (thanks for the plug, by the way) but you have to label it with whatever you put into it. So from a "fix the system:" point of view it doesn't matter, but from a "how do I get rid of it" point of view you still need to know. Also, when checking the ID of a blend against the PT chart - if you have fractionated the blend when it leaked (meaning you lost mostly vapor when the system was off) then the PT chart doesn't work anymore. Another monkey wrench in the process.
The only real way to know is to get a sample analyzed - and there is NO economic incentive to do that. Bottom line - hope that the courteous and professional tech who was there before you labeled the system properly (and think about that before you leave the job yourself...).
You introduced this thread by implying that the old R-12 system was short on gas.....
That was well stated and helpful, thanks!
Your response makes me wonder regarding "how do I get rid of it" point of view. If I'm filling a tank and don't know what it is at the time, but later take a saturated temp reading after tank ambient stabilization, and match it to........something on a PT chart. Am I at financial risk when I bring the tank back to my distributor? I mean if they determine that it's a mix or some other refrigerant blend. I was told they burn it if it doesn't match, end of story. But I've also heard they can now "fractionate it" to separate all the different individual refrigerants via fractional distillation as no2 heating oil is separated from crude. Is this true?