It turned out the speedometer wasn't moving due to the grease that had been used previously, which by its dark grey colour may have been graphite or something similar. It had turned very sticky and almost solid, enough to stop the magnet wheel from turning completely. Other than that, all parts are in fine shape.
I should also add that there was also a small dead spider in there. How that got in, is anyone's guess. Mine is that perhaps the rear opening for the light bulb had been left (or come) open for a while, as it's just a press fit.
In this view, the speedometer's basic components (without the case). You can just make out the spring that holds the pawl to the odometer cog, below the gauge pointer. The wire it's made of appears to be the thickness of a human hair. Magnet wheel is at the bottom.
In the foreground here is the odometer assembly, which is also supporting the drag cup on its rear face. Very critical to make sure this is supported carefully, so the drag cup stays in the position you see here, otherwise the extremely delicate return spring can stretch out and get damaged. In the background at left is the magnet wheel, whose shaft had seized. The brass part on the shaft is the screw that turns the odometer. On the right is the basic frame for the gauge.
Basic assembly back together. You can just see the coiled return spring in the middle.
Assembly with the gauge face and pointer back on.
The speedometer back in it's case, with the inner bezel, glass and outer bezel waiting to go back on. There are hollow rubber o-rings (split but not shrunk - perhaps were never complete loops?) on both sides of the glass. The outer bezel (upper right) was sanded and repainted after this photo was taken, and so the gauge won't be completely back together until the paint is completely cured, which will be in a week.
