It all really depends on how old the speedo is, if it is 1950s then a lot of vehicle types were much slower then than now. Odd that it reads backwards.
Vehicles normally have speedos with a higher speed than can normally be achieved, so I would guess this one would rarely have exceeded 10 mph. 5 - 10 mph is very much the speed range required for goods yards, docks, Mail sorting depots, large factories, stations etc.. When I was young and often going through Waterloo and Victoria I think I remember platforms used to have 5 MPH signs on them, and powered wagons with the driver standing at the front. Sometimes the wagons were joined to form a train. Once outside the station they could do 10 mph.
If it is appreciably older then it could be for old London taxis (particularly the electric ones) or Harrods electric delivery trucks, but it really does not look that old.
