The old 45 RPM (revolutions per minute) vinyl records were called 45's
by everyone who had them.

If we were using a metric system at that time, would they still be
called 45 rpm, or would there be some metric numbers used instead?

I was having this discussion with a few people and no one knew the
answer.....
(Personally, I think they would still be 45 rpm, but I could be wrong).