Every speedometer I have ever seen from this era was an open frame, just a bent strap holding the odometer and cable mounting. Once removed from the dash housing you can see everything, the magnet, the worm gear for the odometer, all the odometer digits...
Hmmm, I never recall any open frame speedo in A bodies; IIRC, they are all closed in on the back with a plastic housing. There might be a way to open them up; I never tried.
