I was told by my gunsmith that the top of the 20 ga 1187 was to thin for D&T to mount a scope on it. It may be different for a dot type sight. I found at Brownells, a Da-mar mount that attaches by 6 holes D&T on the radius area(3 on each side )of the receiver. I haven't ask him about that but it specifically has one model made for the 1187 rem 20 ga.
Flyrodder