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.
Since I'm developing a roll right now, I thought I'd check...
The densitometer reads around 6.0 on the paper backing from a roll of 120 Panatomic-X.
Folded in half, two layers still reads 6.0 so I assume I have reached the limit of the device.
One comment I'd make is that the backing paper and printed numbering used for Ilford 120 films is very difficult to read through the red filter of my Ikonta 531. Unfortunately the Novar lens on the camera is soft it has aslight haze and is very low contrast so unusable, some of the new optical glasses used by Zeiss and Leitz in the 1930's don't age well. Luckily I picked up a new (unused) post WWII coated Novar so will switch the cells over next time I'm in Turkey.
I've been restoring 3 pre-WWII Rollex and a Rada 6x9 backs and like all the cameras & backs with the viewing window it's important to ensure that the light seal between the back and the pressure plate is intact, older models used velvet newr foam.
As Bill's just noted the film backing paper itself is light-tight it's where the film is spooling on/off the roll where you are likely to get a problem.