Those sharp-edges worried me. It looked like it could/would eat the new piston seal within a few shots. Thought about making an insert to fill in near the edge, but it seemed like difficult work considering it's so deep in the tube.
This 1" stainless steel washer should do the trick, but it's just a couple thousandths too large in diameter to fit into the compression tube.
Mounted it on a mandrel and touched the edge with a mill file. In about two seconds, it was sized.
The washer barely drops to the bottom of the tube. (OK, it's technically the front of the tube, but you get the idea.)
Washer is so thin so there won't be any appreciable change in compression stroke.
Used a good degreaser to thoroughly clean the inside of the tube.
Cut a piece of aluminum tube to fit inside the length of the tube when the end cap is threaded down.
The aluminum tube will push and hold the washer against the front of the tube as the J-B Weld epoxy sets up. There's really not another way to secure the washer in place--at least, none that I could envision.
Not using the quick-set version. Want the longer set-up time so I don't have to rush--as well as the higher strength.
A plastic spacer wrapped near the end of a 3/16" steel rod allowed me to get down to the bottom of the tube without the epoxy covered end touching the inside wall.
Washer dropped into place and the rod and end cap installed. I cleaned the excess epoxy from the transfer port and set it aside. I'll look at this again in about twenty-four hours. In the mean time, I'll clean the barrel and degrease the mainspring. The stock could probably stand another coat of oil, too.
Reassembly should happen in another day or two. Please check back. I hope it all works.