Black Tamiya dish wheels have tighter fit than white and yellow.
Can be really tricky to get black dish wheels to sit properly. Never had that problem with white or yellow wheels.
Text imprint direction is same on "new" white and "old" black wheels = Same mould.
On the 501X the "old" white wheels had the text imprint like this:
And the yellow wheels:
So my guess is that the "old" white and yellow wheels used same mould.
Black and "new" white wheels use same mould.
Hexes are 9.7mm, so same as 501X/511 hexes.