I'm answering myself on this one. I guess thats what happens when you get old like me. I called Lyman today and they said that there apertures have  .225 X 40 threads to the inch. Then I called Williams gun sight Co. and theirs is 7/32 x 40 which is .218" in The Midway Master Catalog under the Williams apertures it states that they will  fit Lyman,Williams and Redfield. There is a greater selection of Williams apertures than Lyman.

enough said?

The Merit eyepiece would be a great addition to your sight, allowing easy changes for differing light conditions and target sizes, and it's totally legal for Schuetzen shooting.

However, please be advised that the Merit, or indeed any other iris-type eyepiece, is NOT legal for BPCR Silhouette. For BPCRS most folks use a Hadley-style eyecup that has a rotating disc with a selection of different-size apertures.
Good luck, Joe

PS: this was good info as of last year, but the NRA changes the BPCRS rules almost every year and so it may be different by now.

I use the Merrit disk on rifles that have Lyman reciever sights, MVA's Hadley disk on my Schuetzens as well as an adjustable Anschutz iris on my Smallbore Prone rifle.  With my eyes I need the varable size for different light conditions. Merrit makes a disk that sticks on to your glasses and works very well for sights that cannot be altered.

I know NRA for BPCS has a problem with adjustable disk but if you can't see the target why shoot the match.  I don't think any match director outside of the Nationals will send you home for using one.

If you are going to the nations pay up get a Hadley disk and they will pass muster, Plus they do work very well. 

Boats