I have received a message from Dr. Milos Mladek this afternoon, explaining what this hood is. Here is what he is saying:
"in the last days there has been a discussion on the FED sunshade in your forum. What the
participants wrote is understandeable and logic.
But things do look different when you actually have the sunshade in your hands. It is a one-
of-a-kind design all in all. The hood has a peculiar shape in profile, that can be seen on the
attached images.
What is important is that the hood is mounted by pushing it on the front of a FED f/3,5
f=5cm lens rather deeply until it sits firmly, which is due to a strip of black felt (which is also
visible on the pictures by Haley, a little worn, but still there). The strip of felt is glued-in
where the hood shows a bulging ring on the outside, and the fibres of the felt strip engage
the knurled structure of the FED lens front. In my eyes this hood (that is a copy of the first
Leica hood "FISON" in the first version 1925-1933, which has a rectangular opening on its
front that is not unlike the FED hood) is definitely made for a lens that is rotating when
focused (the FISON also had to be re-oriented as well when the distance had been set).
Why am I convinced that the the FED hood was made for a rotating lens? The FED hood
has a mark on the upper side that gives you a reference how (in what orientation) you
should push the hood on so that the rectangular opening is level. I have made photographs
that show this marking, and when assessed with a strong loupe it is clear that this marking
is old and was made before the hood was painted black. And there is no other explanation
for this marking than the need to adjust the hood so that the rectangular opening is level
once the distance has been set."






Cheers,
Vlad.