Kirk,
My earlier question on measurement was for the width across the frame/watertable flats. From your picture of the two sideplates it is apparent that the 20ga is reduced in height but I cannot determine if it is scaled down in all dimensions. Are the mounting screw and bridle pin locations positioned axactly as the 12ga plate?
I took two of my 12ga., Meriden guns apart to take a look. The first, #3142, is marked "The A.J. AUBREY". It has straight cocking Rods vs. the dog legged cocking rods of later guns like yours. Gun #635xx is marked, "THE MERIDEN FIRE ARMS CO." Both have the two piece firing pins, with no spring and a threaded retaining nut(seat).
This picture shows the early style(straight) cocking rod. Mid length is a retaining screw in the frame.

Both #3142 & #635xx have identical firing pin assy's retained by a threaded nut(seat).

I went back and looked at your picture above and it does appear that the side plates retain the firing pins in this frame.
In the end you may be left to reverse engineering the parts from the frame dimensions. I have an idea on how to modify 12ga cocking rods to work, if the 12ga. "rod" section forward of the offset is the same diameter as your 20ga. frame.
John