Guessing 1/4-28? I counted 14 threads, and .247 is as close to .250 as damn is to swearing I can pull my cooey 75 out and check the threads on its screw but it's a cr#pshoot about whether or not the threads match. Well... that was a dead end. The screw on a 75 is captive, closer to 3/16, and finer thread. Couldn't get the thread gauge on it (not that it matters) Sorry, dead end.