Thank you. That was a use I did not think of at all.
(For the quite similar holder F for the F-1, though without height differences, that use is not even hinted at in the semi-official german F-1 handbook.)
For the even less initiated: that holder kind of braces the front of the body, thus taking off bending from the base due to a long lens. But that brace at least partly is also needed to fix the camera when resting vertical even without long lens.
But still that does not explain the different thread hole distances.
And that was what puzzled me...