Mike-
That is beautiful work there.
The Stanley's used a two piece piston.
I am not sure about the Mason.
Worst case, you can replace the pistons with some that have cast iron rings. I have had trouble with machining many many things, but I can actually make cast iron piston rings without much trouble. I just turn off a few extra in case I break one, but they are not that bad. I don't do the heat-treatment, but turn them oversize, cut the gap, spring them in on a mandrel, and then turn them to final size. I make them slightly wide, and then sand the sides on a flat plate until the fit into the slot in the piston with a little to spare.