Where does the conversion happen? It looks like the speedos have the same degree of needle sweep range. Does this happen internally w/the speedo itself?

Maybe no conversion, but just different dial faces - with the needle in a certain position it reads 'X' mph or 'Y' kph.

I bought a RD350B where there was clearly a replacement mph stick-on dial face on the speedo - if you looked closely you could see the original kph face underneath.

You can buy both , Km/h and Mp/h decals, but both Speedoneters are different. The day trip and Km counter slots do not align. Inside, the clockwork of the day trip meter is different as well. Finally also the knob is different, as well the position, where it goes out of the housing.

Fair point Wessi - there would have to be a difference in gearing for the mileage and trip meters.