Tom, I have run into this before and I just think it is a tolerance issue. If you don't mind taking apart the brake and getting the bolt barrel so that the threads can be placed in a drill chuck. You can take a piece of emery and hold it with your fingers around the barrel to reduce it just a little bit and try to fit it. Do this over and over till you get the fit you want. If you are worried about the threads in the chuck then wrap them with tape first. You don't need a lot of force on the chuck to get the desired results.
I know some folks are reluctant to break down a set of calipers as they can be a challenge to re-assemble, especially with arthritic hands. To me that is easier than messing with the frame and cheaper to correct if you make a mistake.
Elizabeth, New Jersey, USA