I recall; miniature incandescent bulbs embedded into push switches that could display/set registers, it was very important to test the bulbs before trusting the

I recall; miniature incandescent bulbs embedded into push switches that could display/set registers, it was very important to test the bulbs before trusting them (a long press) else a burned out bulb may catch you off guard. Right next to the core memory cabinet, someone accurately patched it to show huge "$" during idle time!!