The modern TO-247 has a notch on each side which exposes the leadframe.
I think the leadframe explanation is the correct one. Packages like the TO-220, which have an exposed leadframe, sometimes have notches and sometimes do not, but all of the packages with a hidden leadframe seem to have notches exposing it.
Modern packages, and the TO-126 (TO-225) and TO-127, have the advantage over the TO-220 of not requiring insulating hardware for the screw attachment, but without an exposed leadframe, some means must be made available to fix the leadframe in place during encapsulation.