Dodge City.
I had seen plenty of 870s drilled and tapped, so I just hauled off and did it myself and mounted a Mueller Quick-Shot sight. The 20 gauge doesn't generate very much recoil, even with turkey loads, so the stress on the screw threads (6-48) is not excessive. Apply a drop of the blue Loc-Tite to the threads and tighten them to "firm." If you have a torque screwdriver or wrench, that would be 15 inch lbs. Rule of thumb for the 6-48 screws is torque as tight as you can hold the tool with just your thumb and 2 fingers. Also insist that the screws be made of steel and not the cheap aluminum types.
Bottom line is that it can been done, has been done and will continue to be done.
Cheers
FullChoke