As a final follow-up on this issue, I sent Remington an email asking if the radius on the top of the receiver was the same on a 20 gauge as it is on a 12 gauge. They responded that this radius was the same, but that the 20 gauge frame was smaller.
This means that a sight or scope mounting base that is designed for an 870 shotgun, unless it clearly states otherwise, will fit both 20 and 12 gauges.
FullChoke