Bob Kirkpatrick wrote:

Thanks for the information. I've looked through some old catalogs now
and I can see the difference although it's a little like the kid's
puzzle where four of the the six dogs are slightly different. It
appears that another change is the addition to later models of the
shutter pre-release/mirror lock-up button under the winder knob.

Not so.
All 500 C(...) models have that button, though it was a bit shorter (and
perhaps harder to see) on earlier models.
;-)