Icing is indeed an indicator of a low charge. If the tag says that R-12 is in the system, it is probably still using R-12. Small critical fill systems like this are generally sealed systems that rarely need service.
BUT... the only SURE way to tell which refrigerant is currently in the system is to have it analyzed. There are analyzers on the market that can tell you which refrigerant (or mixture of them) is present.
Since the capacity is so small, it would be critical to have the right amount in the system for proper function. Recover, vacuum, and recharge with an accurate measurement is the recommended course of action. Then you will know that it has the correct charge in it.