I have some white plates with square holes on the underside. I'd almost think they were some imitation brand if they didn't have lego stamped on the studs. Can anyone tell me the history of when square hole plates were produced and for what purpose? Is this just a really old design? Why were they changed?
Those would be Waffle Bottoms, which seem to have been mostly phased out by 1969, except for the 4x8 plate that went until 1971. Having not read any deep history on old LEGO designs, I'm going with that was just what they went with to start, then realized that the tubes were much better.