I have two Williams FP sights, one that I have had for about ten years and one recently obtained. The newer one had a aperture larger than a like (about .120"), so I tried to swap one from my other sight but it wouldn't screw in. Also another FP aperture I have wouldn't either. The aperture from the newer sight does screw into the other sight but the threads do seem a little undersize. Did these sights come in two different thread sizes. All three of my apertures seem to be the same TPI. Also, checkering on apertures are different, my two from my first sight have crosschecked "lines" while the newer one are "V" cut crosschecked like found on Marbles apertures or on rifle stocks. Not sure what I have here. Any thoughts (if I didn't totally confuse you).
Thanks,
Dave.

I haven't owned a lot of Williams but the ones I've seen all had the same aperture threads.

Apertures for Williams, Lyman and Redfield sights interchange so you could have any combination.

The Marble's uses a smaller thread than the others.

The shank of the aperture disc is threaded 7/32-40, same as Lyman and Redfield. I have used the Williams "twilight" disc in Lyman sights so I know the threads are the same. I was not aware that Williams used two different thread sizes, but my experience with WIlliams receiver sights is limited to 2 or 3 that I purchased in the late 70's.