Williams, Lyman, Redfield, and other classic US sight eyepieces use a 7/32 X 40 thread.
German sights standardized on a 9.5 X 1.0 MM eyepiece thread by the 1950's. This facilitated an interesting aftermarket in accessories that will fit many different guns.
A really nifty thing to have is the inexpensive adapter bushing (sold by Champions Choice and others) which allows Euro disks to fit US sights. Here's one in action--an old HW 50 rifle, with a Williams sight sporting a Gehmann adjustable iris:
(As an aside...the fabled British Parker-Hale sights are also 7/32 X 40, though the steeper UK-standard thread angle makes the disk shaft slightly larger in diameter. Thus an odd situation where US disks fit P-H sights, but usually not the other way around. The exception is Lyman sights, which will usually take P-H disks.)