I just checked a Weston Master II, IV and Euromaster with a two-piece incident light attachment (cone + filter) and the later 1-piece type. Master models up to III need the 2-piece type, which is (just very slightly) smaller than the later type, so will not fit later meters without falling out. Conversely, the later Invercone (a conflation of the words INVERted CONE) will just not push far enough into an earlier meter to lock into place. Note: When removing an Invercone, push it back towards the baffle hinge to release it. If you don't, a small plastic nose on the attachment will become worn and it will not then lock into place.

If you want a Weston for incident light readings, Master IV and later are the best bet, since they have a pointer lock. With earlier models, you need to be able to see the dial at the moment you take a reading, which is a bit limiting.