I decided to try and drive the rivets out with a punch and they came out without a fuss, the mechanism is pretty simple



Here is the back side of the screws holding the trigger group assembly to the action, you can see they have been staked in place



I figured they would be hard to get out and I was right, I tightened, then loosened, tightened, then loosened...finally they broke free. They had been screwed into that receiver since 1948/49 and never came loose.