If I recall, the big honking plugs were called "Joy" connectors. I think it was the manufacurer's name. I was on submarines, we pulled 4 400 amp 3phase cables into the escape trunk where the recepts were. Ours were threaded. You used a huge spanner wrench to turn them. Ugh. Bad memories.
The Navy used the 400hz sets on the computer systems. It was a method to speed up computing speed.
