I don't know what their thinking was on pre WW II M70s with receiver ring only drilled and tapped. Maybe they meant it for a rear target scope block, with the front out on the barrel. I have pictures of that installation. Maybe they meant for the purchaser to have his own holes drilled, there were so many oddball scope mounts in the early days that there was no standardization.
The holes on the side of the rear bridge are really for a peep sight, although there were some early scope mounts made that used them along with the top holes on the front receiver ring. Maybe you could find one of those old Stith mounts.
Having holes drilled and tapped in the receiver bridge for a modern rear scope base will knock a good deal off the dollar value of that era rifle. You have to balance that against how much you want to use it. I have one that was drilled and tapped, but the price was right and it still shoots well.