Jerry, I am trying to have a shed clean up(to make more room
) and get my 78 Magnum out at the weekend.
The width of the spacer isn't that important to the wheel(hub) positioning between the fork sliders.
It is the width of the backing plate that is clamped between the drum side of the hub and
one fork leg that determines the position of the hub between the legs.
The opposite leg is just gets clamped onto the large axle boss OD after tightening the axle. They could have just made that axle boss longer, but then you wouldn't be able to swap the spacer out for a GS speedo drive of the same width if desired.
Saying that though, I will try to get you those exact measurements of mine at the weekend.