I can't speak to the stainless gun BUT the model 19 2 1 /2 and all the other longer barrels were different at the front of the frame where the rib matches up. When we were doing Smolts it was important to start with a 2 1/2 instead of a longer tube to get the rib to match without a space. May not be the same on 66s. It is very easy to round to this configuration and is very good for people like me with small hands.