[Brightcolours]

I'm not sure about the need for stupid xx.5mm increment marketing focal lengths.  Couldn't they just call it a 43mm lens (both Voigtlander and Leica)?

 

This marketing number is surely more precise than the nominal (as-designed or as-computed) and actual (as-manufactured) focal lengths.  

 

It could be acceptable on small-format wide-angle lenses where each 1mm translates to a more significant difference of field-of-view, but on a 43mm lens, it's only a 1% difference!  

The reason they call them 42.5mm lenses is to make totally clear that they are 85mm FF equivalents....  Traditional 135 format focal lengths are the "standard".