Rocketman, if the 90 degree angle is formed by both sides of the cutter, it seems to me that the 60 degree cutter is the one to use on most older guns. Their checkering was usually left flat topped and the 90 degree cutter, seemingly more modern, creates sharp checkering points at a shallower cut than the 60 degree cutter will. Lots of vintage guns ruined with New Sharp Pointed diamonds in the checkering.

http://www.bakercollectors.com/