Morning John, sorry I misunderstood your request. Guess I need to google a firearms terms dictionary. On your original question, the dimension across the 20 ga water table is 1 7/16" and both 12 and 20 sideplates are 4 1/2" long.
Yes, the mounting holes are the same and bridle pins are in essentially the same location from front to back. However on the 12 ga it is up 1/32" higher from the bottom of the sideplate. Following are a few more pictures. The first shows the 20 sideplate sitting on top the 12. Everything matches up except for the height.
Couple other pics showing the two sideplates. In both the 12ga sideplate is on the bottom. Another difference between the 2 is that the 20 sideplate is .08" thick while the 12 is .125". This difference makes the 20 sideplate weaker which can be seen by the bending. Also, the hammer on the 20 is 1.06" tall and the 12 is 1.17" tall. You can also see the difference between the bridles.
The firing pin setup is as I expected. This post is getting a little long so I will provide some additional questions/info/photos in another.
Kirk
