This is probably the key to the issue.

I use a Gaoersi 6x17 and that has a clear window, very much larger & no red filter and I get no problems even when I've forgotten & left the cover open and I'm shooting in very bright sunlight, meters virtually at it's highest.

My two pre-WWII Rollex roll film backs have a felt shield that acts as a baffle to prevent light spread sideways from the frame counter widows.

I've just looked closely at my Ikonta 521, that has felt seals as well, they were OK but the pressure plate was a bit weaker than my other cameras.

My experience with velvet/felt seals on older LF cameras is it just flattens over time & use and can become quite ineffective. On roll film cameras these felt/velvet seals are designed to stop light passing underneath the pressure plate which has holes in it coinciding with the counter windows.

So I'd suggest look closely at these as well as the pressure plate.

Ian

Quote Originally Posted by bblhed View Post
I do have a tip for you that I didn't see posted, make sure that the backer plate that holds the film against the slides on the bellows is doing it's job and is still springy, in cameras of that vintage they were sometimes made of Brass and can take a set over time, you may need to just give them a little tug to be sure they are holding the film correctly. That was the biggest ruby window light leak problem I had until I figured it out.

I use the electrical tape window cover as well, I do about 1 and a half turns of the winding key and then look to get the number into place, I also shade the window with my body as well. No problems any more.