Not sure about wether the trigger mechanisms were in different sizes for the frames sizes. The hammer would be the smaller size in a 20ga frame gun.
The only thing done to the hammer if I'm recalling correctly is reshaping the back edge of it. That edge is in effect a cam surface that pushes that arm back to reset the mechanism.
The same set up was used in the NID SNS trigger.
I have a N/Special 20ga Skeet w/ SNS trigger somewhere. I'm going to have to pull the stock off it now and take a look at it just out of curiosity again. It's been a while since I worked on one.
If I can find it, I'll take a couple pics of the mechanism.
