i don't know what manufacturer would think a window of any opacity other than 100% would not fog modern emulsion film
But in fact these old cameras are generally well enough baffled that there is little problem with fogging modern films unless you've got fast film and direct sunlight coming in at the right angle to strike the inside of the camera and reflect up from the other side of the pressure plate. Modern Chinese rollfilm backs often use a red window, or as I mentioned, a fully open window with foam light seals between the shell and the pressure plate, as well as metal shutters that should be kept closed between winding.
When cameras started having frame counters, the red window might be used to find "1" and then a metal shutter could rotate or slide to cover the window, or in some cases there are cameras without a frame counter, but a red window and a shutter to minimize the possibility of fog.
The window system is actually not a bad system. Frame spacing is absolutely reliable, unless there is operator error, and a camera or back with a frame number window is lighter, less expensive, more compact, and has less to go wrong than a camera or back with a mechanical counter. It is also easy to use masks and multiple windows to create a multi-format back with the window. To do multiple formats with a mechanical frame counter, like with the Sinar Zoom backs is a much more expensive proposition.
