The Stovebolt Site Forums The Shop Area The Electrical Bay Tachometer

You came up with the name "D'arsonval Movement", so you already have a pretty good idea of what you have there. There is a permanent magnet in there and when current flows through the coil winding, the two magnetic fields push against each other and the armature turns and the needle with it.
I expect that there is a tiny delicate coil spring on the shaft or even one on both ends. A clock repair person might have an easier time working on it.
I wonder if a sonic cleaner would clean it up without damage.

I have emailed you the wiring diagrams of the corvair tachs. I would guess that they shared the same circuitry. The Corvair used a 1 milAmp meter movement.