When we are talking the difference of 1 to 3 degrees to see if a 4" pocket is cut differently than a 4.5" pocket, there is not a picture in the world that anyone here can take that would allow a measurement that accurate. Get that out of your heads. The only way to measure it would be with a physical angle gage held against the pocket.
I do recall Glen mentioned several times over the years that the tighter pockets are cut more parallel than the loser pockets. I can't recall if he said that's how Diamond does it, or if that's how he personally does it when extending a customer's rails in their home. Probably both, knowing him.
I do have a bevel gage, I'll try to bring it to the hall this week. My local hall has 4.5", 4.25", and 4.125" pocket 9' Diamonds ordered that way from the factory.