My favorite transistor computer was Seymour Crays’s awesome CDC 6600 sporting 400,000 transistors, 100 miles of wire and Freon 22 cooling.
In the pre-LED era, one of the interesting aspects was the almost total lack of small bulbs compared to other medium and large systems. All status items were monitored by a peripheral processor and displayed on one of two large CRT displays.
In the pre-PROM era, the bootstrap for one peripheral processor was memory mapped to a series of toggle switches hidden in an end panel. I never heard it referenced as such but in today’s terminology, I consider it as a Mechanical Programmable Read Only Memory (MPROM). The switch settings formed machine language commands to select and read from the boot device which was typically a tape drive.