Also, I know nothing about the practice of widening BB holes to obliterate distinctive brand cut-outs and serial numbers (except what I've read in this thread), but I'm struck by the seeming irregularity of that cutout, and the size. It is so artless that I can't imagine it being the original cutout.

If that's true, that there was a distinctive cutout that was cut away, is that hole big enough to obliterate the Masi "M" of that era? And didn't Masi put their serial numbers outboard of the cable guides? Here I'm just going off a few google images, but that BB doesn't seem to fit.