I am working on one myself these days. Actually, this is the second
The piston is threaded onto the piston rod, so head space is already adjustable. I was just being cheap and simultaneously trying to keep as much of the original parts as possible. Speaking of head space, I was able to take an impression of the end of the cylinder (pump outlet into valve) to see what we had down there. There is no cone shape like later valves. It also isn't just one piece, like there is a valve body and what seems to be a largish screw-in retaining ring (two slots cut in it). There is a very small groove between the center area and that outer ring. (The impression of one of the slots came out pretty ugly.)