What were your pressure readings before you pulled off the gauges? If it a few seconds of refrigerant escape tripped the low pressure switch, the readings probably weren't worth reporting.

If you could find a shop with a refrigerant identifier and confirm what refrigerant is contained therein, you may be able to proceed by simply topping it off if you can't find any leaks. In your climate, 134 might very well get the job done. If not, you can flush and reverse convert to R12.