The Chronometric speedometer / Tachometer is a principle commenced by the Jaeger speedometer company in Paris, France, in the 1920s and as such is a METRIC instrument, that is the dimensions, screws, threads etc are metric and not imperial as generally thought.
For example on the motorcycle instrument, considered to be 3” dia., with a 26tpi thread, the bezel is 80mm dia. x 0.9mm thread. Sometime in the late 1920's, Jaeger Paris set up a company in England, Jaeger Ed., with two British directors and one French director. Around 1927, Smiths Motor Accessories purchased this company and re-named it British Jaeger.
For a long time the dial faces, either with the British Jaeger or Smiths logo on them, also had a credit to Jaeger …”Jaeger patent”. Smiths often badged their instruments Jaeger or Smiths.