A, it was neither painted or plated from manufacture. Perhaps bead blasted and then shipped.
B, it is most definitely NOT an impact hammer. It is as you noted very similar to a 2 cylinder engine. I believe if you look close you'll see the throws of the crankshaft are not 180 apart either, but not far off. That prevents the wrench from getting stuck with one piston on top and the second on bottom.
My hunch is the tool was developed from the rotating mechanism on a steam drill but I have no way of proving that. The engine part of the tool will continue to run until either maximum torque is reached on the wrench, or the air pressure coming to the tool can no longer force the piston down the cylinder. The control of torque is accomplished by either the operator or a pressure regulator on the air supply.
These wrenches were primarily used for assembly, not disassembly. They can easily break Mack U bolts on the old camelback suspension. Since U bolts are removed with a torch the wrench was ideal. They were also popular building heavy machinery in the time of men with strong backs before OSHA.
This in BOT a 1 man tool unless he can block the handle and is damn sure he can let go of the trigger. Because of that, and because of the arrival of 3/4 and 1" impacts that were much lighter, the old piston drives never made it when steel erection went from rivets to bolts.
BTW, you are going to need minimally 1/2 hose and 15 to 20 cfm of compressed air, and a good supply of air tool oil.
