Oh my goodness! I am TRULY sorry for the late reply.. Still kinda new to PF's console and what-nots.
Hope i am not too late on this. But ok, pertaining to the availibility of the FS, you will need to drop them an email enquiry on it and they will "make" it for you and list it up as available.
As for the report that it will get darker, well, yes.. Just slightly, from any aperture smaller than f/3.5.. It can get darker than the stock screen's, at all given aperture past f/3.5..
As i do not have that much experience with M42 lenses, i wouldn't know how to advise you on this. But if the lens focuses wide open, only to stop down at the desired shooting aperture when you finally hit the shutter button, then it should not affect you at all. Unless of course the largest aperture of a given lens begins with an aperture smaller than f/3.5, then i would be slightly concerned.
Know what i mean? And this would be very much similar to the KatzEye too..