On Thu, 31 Aug 2017 12:00:46 -0400, wrote:

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?

The metric part of the world also measures time in oddball units. I
think we are stuck with it. Switching to a new system would be
essentially insurmountable.

In the SI unit system, however, the unit for angular velocity is
radians per second. 45 RPM is a touch over 4.7 radians per second.
--
RoRo