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21 bits ... allows front panel access to more than the 16 bits of the machine registers. smokin'!

The /70 had a 22-bit address bus, so I assume that switches 16-21 didn't have any effect unless you were entering a memory address.

I read something that said Ken Olsen found these switches on an old model washing machine and thought they would be great on the PDP machines.

I think it might have been the 1957 Philco-Bendix Duomatic washer/dryer front switches that inspired the PDP-11 panel.