Thanks for taking those pics, many b's. Nothing's been cut off, thankfully. Typical modification involves tapping that hole, which originally is smooth bore, and grinding a notch in the outside radius of the hanger, to act as a stop for derailleurs like the Nuovo Record, etc. Yours hasn't been ground, however. A good thing.
I'm not sure if Sun Tours require the notch. What derailleur was on the bike when you found it?