These inserts sit in a slot behind the trigger and, after adjustment, are secured in the frame with a screw. The screw threads into the body of the stop itself, not the gun frame. Early ones were shaped sort of like a football, with the screw hole off center, and were prone to coming loose and interfering with the trigger's movement rearward. Because of this, they were often removed by armorers to prevent this from happening, particularly in duty guns.
Interestingly, the screw used to secure the stops is the flat-head screw used in the tang of the old rear sight assemblies. I have accumulated a slew of them over the years.
Carter
