I found a 1940s compressor with a piston head for a friend of mine, provided he checked the tank. It was internally coated with what looks kile epoxy paint. There was a teaspoon of rust in the bottom and visueally the tank looked brand new.
I bought a 170s Atlas Copco compressor, and the tank is more than 180 thou thick - even so I'll be running it at 90 PSI instead of the 175PSI it was originally intended for (the test says 250 PSI for operating pressure, I think) As why the reduction in pressure? Because of the greatly lowered stored energy. Faster refils, less dangerous if something bad happens.