Louis, everything inside the speedometer is mechanical. Since the odometer still works, the cable up from the transmission is still rotating properly. What you can see as a speed indicator from the driver's seat is a long painted drum lying on its side. It rotates through part of a revolution in an arc proportional to the RPM of the cable coming up from the transmission. A spiral paint band on the drum makes the pointer look like an arrow through the viewing window.
This type of speedo was more troublesome even in the old days than the more conventional visible-pointer type (cost about twice as much too). The drum bearings may have worn out and the drum cannot revolve freely any more. Drum might be stuck from dirt or have a broken return spring also.
The complete head can be removed from the dash for repair. The cause may be something easy once the case is opened up for inspection. Unless you like working on delicate mechanisms like clocks, it would be best to send it off to a specialist dealing in speedometer/instrument repairs.
What has been, can be again. (Bob Wills, 1942)
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