Good answer
It is possible that the Martin's soldered on toneholes rather than "extruded" (pulled out from the body) had to be thicker in order to have enough surface to get a good soldered contact with the body. This thicker ring if left as is would produce a tonehole with a rim far too wide to be practical for seating pads---hence they were beveled at the top to make the rim a more acceptable thickness. I don't know this for sure, but it is my story and I'm sticking to it until somebody comes up with a better explanation. ;}