What was the slot for in the 1950's slotted bricks? Bricklink has a list of ancient bricks from the 50s that have slots in them. What was the purpose of the slot? Its positioning on various pieces suggests that there was something specific you were supposed to put there &mdash; a window, maybe?
Oh, I should have read further down the page. There were special panes of "glass" in frames that fit the slots.