Skeet 028, I just noticed the last part of your post. While they are nowhere near as common as they were 5-10 years ago when I was in my building mode, K22 and Model 17 six shot cylinders are still pretty available from all of the normal secondary market sources. I’ve also found Model 48 cylinders (22 Mag) show up pretty often and work just as well. Any of these should be pretty easily fitted to your 16-4.
Froggie