1920's ?? British SIMMS electric speedometer-what does it fit?

Nothing specific on your actual unit i'm afraid, but Simms were suppliers of proprietary auto-electrical equipment (magnetos in the main i think), so it could have been fitted to any number of the myriad car companies around at that time.

I did a UK Google search on "Simms Motor Units" and it brought up some quite interesting stuff.

Hope this helps