Oh, all those machines used linear ECL (Emitter Coupled Logic) circuits for speed. Terribly hot logic family but complimentary FET based devices just couldn't yet cut the mustard.

FWIW, the transition from the Model 155 to the 157 heralded the introduction of table driven virtual memory. We called it "relocation" hardware then. :-)

The first engineering model of the 155 used core memory but transistor memory became fast, reliable and cheap enough during its development that a very disruptive re-design was performed across the whole family of machines.

That was my first job out of the university and what fun it was! And what an incredible experience working for IBM was then.