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So just for the one nine year old ITT:
The T-72 for example has a magnetic memory unit that stores what ammunition is placed into each cassette, and spins with the autoloader carousel. So when the gunner selects say HE on his ammunition selector knob, the carousels starts spinning and when the memory unit reaches the first sector that corresponds to a cassette loaded with HE the carousel stops.
Then depending on the firing mode it either loads automatically, or the gunner presses the load button to initiate the sequence.
This was in the early 1970's.