I have the Williams peep sight , tried all the different aperture sizes , Twilight , long shank , Merit adjustable #4 but if your going down that road I'll save you the trip. Go straight to the Gerhmann Redfield/ Parker Hale adjustable iris .5mm to 3.0mm , may cost alittle more but worth the difference , for the Williams get the one with the 7/32 x 40 thread . Merit will do the job but you don't know the aperture setting and adjustments are rougher to set , the Gerhmann is marked and adjusting is very smooth . The OD is larger then the Merit also . Hope this helps if your looking for a good adjustable aperture for your peep
Chris
This is a bunch of helpful data I found....