Septic clean out cap

[Mike 63]

The clean out for my septic line in my basement had a brass cap that was not the normal threaded cap. It was a 1/4 turn interlock or something. It wouldn't seal so I put a rubber cover over the cast iron hub. It's worked for years but I'd like to replace it now. However, I can't find anything like it. Everything is threaded or expanding. Anyone know anything about these? When I first took it off years ago, I seem to remember there being 2 sizes but now nothing comes up