John as far as I can see, the CZ26 had a black face and was for a car/truck 20-0-20 amps. I would hazard that someone has replaced the face in yours.
Ron
I thought of having a go at doing that, as the 20-0-20 are much cheaper. It wouldn't make much difference on the bike, I think the CZ27 I've got only works when it wants to.
Strange but my 1941 wm20 also has a amp meter with a white face ? , it seems to have been on the bike a long time and appears untouched and not messed with
I can't claim to understand the differences, but this list purports to show the motorcycle ammeters fitted 1936 - 1939.
There were differences for magneto or coil ignition. Black, white or Ivory faces were listed...No sign of a CZ (Centre Zero) 26 though and if as Ron says its 20amp then it won't be a motorcycle fitment.