If you really want to find out, plug all the open holes for the refrigerant lines and stick it in a bead-blast cabinet - As the paint comes off you might just find a stamped part number in the crankcase or head casting, or a stamped spec plate on the side.
You may be able to use it as an air compressor BUT you need to flush the oil in the crankcase and replace it with a mineral-oil based lube.
Note that for refrigeration they often have the oil circulating with the Freon through the whole system - but it's a sealed system so it works it's ay back.
The incoming refrigerant suction line goes through the crankcase first
- where it drops the oil and any liquid refrigerant before the gaseous refrigerant goes through the cylinder. And they don't worry too much about the Oil Control Rings letting some oil slip by and go out with the refrigerant. It'll be back.
This makes them less than optimal as an air compressor. Any dirt in the incoming air gets into the crankcase oil on the way through, and as the rings wear they tend to spit a lot of oil out with the outgoing compressed air - To the point you have to put an oil trap in the outgoing line to catch it, and direct it back to the crankcase before it runs dry.
Go find a purpose-built air compressor head, save the hassle.
This one (if it's the right size, and you can get new seals and a rebuild kit for it) would be good for rigging a Home AC Off-Grid, to run from an alternative energy source like a Pelton Water Wheel or a Bio-Diesel engine running this compressor and a small alternator for the lights and fan.
I pulled a two-cylinder version very similar to that out of my uncle's shop after he died. It had a leather flat-belt drive and he had converted it to fill the enormous bomb...er, air tank...that hung from the ceiling of his shop.
A lot of the really OLD refrigeration compressors were set up for thiings like sulfur dioxide, methyl chloride and ammonia. Freon is relatively new as a refrigerant. The oil may be contaminated with some undesirable stuff, hazardous waste. Ammonia takes a LOT of pressure to liquify, so if it was designed for that, delivery pressure could be significant. Might make a pretty good vac pump, too, with proper clean out.