When I got my Moskva 5 someone mentioned that I should keep the window shielded as much as possible with faster film, even with the metal slide closed. I made a not very reliable lip with gaffer tape and tried to cover up the camera when not in use. But frankly I've never seen any effect of it.
That being a Moskva it is plausible that a Zeiss Ikon would be even more reliable in that respect.