"The Deezil prop driver "is" a flat washer with a "D" shaped hole."

Cool! It looks like I've got the original prop driver, then. This engine is getting more interesting all the time. I sure appreciate the info you all have shared. Here's a few shots of it disassembled. The needle valve removes from the brass collet very easily. I'm supposing that the knurled section is supposed to be holding it in there tightly. The backplate gasket looks to be in good shape. The piston/cylinder fit is pretty nice as well. I just need to get the contra piston unstuck.