My home was built in 1959. The clean out cap in the basement is 2.5" ID but it doesn't have threads. It has 2 nubs (now it only has one) opposite each other that fit into two notches in the pipe. I'm assuming you place it and turn to close it. I found it today laying on the concrete floor, one nub is missing and the whole thing looks pretty corroded. Can I replace the cap, I've been looking online and don't see any that look like mine, just threaded ones.
If the pipe itself is in good enough shape, you could just use a compression cleanout plug (sometimes called a mechanical cleanout plug) or a pressure testing plug:
There are quite a few different styles of these, but almost all of them work by compressing and expanding a rubber gasket inside the pipe. 2.5" might be hard to find at the big box stores, but they are available numerous places online. The main consideration would be finding one that would seat deep enough into the pipe that it would be beyond the notches for the old cap.