Do the aperture disks used on Lyman receiver sights (like the Lyman 48 or 57) also fit other types of sights like tang sights? I wonder if there is a universal thread size for the aperture disks or if each brand of sight has its own thread size.

7/32" X 40 or #12 X40 is common hope this helps

There is a guy who sells aperture discs and other small parts on e-bay. I've seen them for Lyman Redfield, Marbles etc. They vary in cost from $10.00 to $20.00 or thereabouts. They are listed under vintage gun parts or something close to that.