The set screws do not seem to have anything to do with some type of barrel to frame fit adjustment. It is a 2 piece setup.....like a plunger with a set screw behind it. This setup allows the plunger to move if the set screw is loosened a bit. Therefore, I definitely think it was to secure something in the chamber.

Chamber diameter measures close to .700, which is normal. Chamber is 2 3/4 which at first I felt was lengthened, but the forcing cone is only 5/8". Normally I have seen longer cones in chambers lengthened later. MBWT at end of chamber is .100 or better.

Maybe....
 

This would be a cool feature when hunting out of a pit or layout blind. Always tough to break the gun without losing the shells when in the position.

Here's a WAG. Looks to be an ejector gun. Perhaps you would tighten the screw if you didn't want a full ejection, therefor preventing the spent hull from hitting the ground.