In Brophy's "Plans and Specifications of the L.C. Smith" it shows a drawing of the screw that Brian is talking about. It is a 7-32 thread and must have been a shop made die as I cannot find any information on that thread even in my Machinist Handbook.
The drawing is from 1918 and I don't know when they stopped using it but on the later guns it does not appear.
A friend had his for-end loop fall off and looking at it there was no broken screw or a threaded hole.
The loop is not dovetailed but has a "V" shaped area to rest on the curve on the top to rest on the barrels.
