Here we have power line frequency meters that show deviation. It's a
funny little gizmo that is actually a small comb like device. What you
see when you look at it is only the tip of the tooth. The teeth are
arranged so that there is a slight difference in resonance between each
one so that only one will move to show actual Hertz. Very accurate too.
From what I remember it was very stable around here.
There is one more point to be considered, that has a far greater effect
on speed of tape in those machines and that is the diameter ratio of the
two spools. Ampex tended to slow down just a little bit toward the end
of a 7" reel. You wouldn't notice it on regular use, only if you tried
to splice parts from the beginning to the end of the tape.