Suzuki as a company dont waste parts and did or probably still carry some parts over to the next year. This was more evident in the 70s. I have no doubt that some bikes were sold in 74 with MPH clocks.
They didnt change many things between the models apart from paint and decals but there are some very minor parts that did change like the gas cap pin going from a spread end peened rivet style to a circlip. The speedos were originally melt sealed then they went to a green glue. this is reflected in the part numbers sometimes with a suffix being added.
34100 is the base code for the suzuki speedo
34100-33618 is the GT550L speedo that it shares with 17 other models
Now the interesting part
There are 16 variations of that number all quoted to suit your model bike. Many of the variations are for changes in supplier of components or face plate design or colour or destined market Im sure one of them is also for KPH
The statement of It must be US if its MPH seems a bit naive when you look into it,
Best way to check is find the original importer of the bikes in the 70s and quote them your frame number and they just maybe able to tell you when it was imported. Not sure but I think it was Mick Hone, Leith should find this topic and he will know who it was.
Cheers