My dad had a college friend who did this back in the 70s, using all electromechanical components. Relays and solenoids and so forth. Hey, they were engineering students, what do you expect? From what he described it didn’t use a drum to display the messages, but rather had an array of six or seven of them that popped up from below, like bread from a toaster. It fit on the platform between the back seat and rear window of his car.

As the story was recounted to me, he did get hassled by the cops for this at one point due to flipping up one of the more expletive laden signboards at an unmarked patrol car. Ultimately they couldn’t find anything to nail him for over it, though.

Meanwhile, my brother in law keeps one of these in the door pocket in his truck:

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