There is only one way to know for sure what refrigerant is in the system and that is with a Refrigerant Identifier instrument. Most any automotive a/c shop will have one. You DEFINITELY want to determine what refrigerant is present before servicing the system.

By law it should have fittings if it is other than the original refrigerant, but I have seen NUMEROUS systems converted without installing the fittings. It is very irritating because if I were to draw the wrong refrigerant in one of my recovery tanks it contaminates the ENTIRE tankful of refrigerant.

Have it identified, then check pressures and go from there.

Good luck,