John--how is the body of the breech set up held onto the cylinder? Is it and the outside of the cylinder threaded or is it silver soldered or something else?

How old do you think the gun is?

Do you think it has been made from scratch or some bits modified from an existing gun?

I was looking at the piston and wondered if that came out of a conventional pistol and then it was reversed so the sear hole was at the front--------and a threaded insert made for the piston head-------------with the original head (which would be at the back now, cut off). I dont know of any guns with a piston like that in a small diameter. You mentioned a bore of about 22mm. I think that is just a fraction bigger than a Webley Junior / Diana 22 rifle.
I also wondered if the housing containing the detant was originally a breech block
There are some very nice touches with the gun and it does look like somebody put a lot of thought into it.

What are the dimensions / weight of the gun?
Thanks