The way I remember it, from an old magazine article, is that Ed is right. They hammered sheet metal to shape on a jig, then smoothed the whole thing with Bondo. So, while they are all supposed to be identical, they all have slight differences. Reduced to N, or even HO, those differences wouldn't be noticeable.
I imagine the construction method did make repairing damage harder for the shop crews, if they wanted to get it "right".