Gary,
I'm going by what the people at Lyman told me. A 1-64 may be the correct size though it may be an 0 or 00. At any rate it is a tiny screw with an odd ball thread pitch that was likely much more common a hundred years ago when the sight was designed. The screw head is slotted and a very short cylinder and flat rather than flat with a conical bottom like most flat head screws. I suspect a round head would substitute if necessary. Steel or brass would probably suffice in this application. If Bill doesn't have an original I would appreciate one of your's.
