Bob,
I use #661-104-004AH (4mm) sight bead. $13. N.E.C.G. Mamoth Tusk ivory (so according to the catalog no international trade restrictions are violated).
http://www.brownells.com/shotgun-par...?sku=661104004
Pg. 282 of the 2012-2013 catalog.
I was wrong about 2 sizes.
"Undersize 6-48 thread, secure with epoxy adhesive."
I would say that the unaltered hole is probably 3-56. If it has been retapped to 6-48 you are in business. If not you will need to remove material as I described above to make it fit the factory hole.
Jerry
Threaded Mode