I picked up this focusing screen "Y" 10 - 15 years back. This screen is for the original F-1 and F-1n. It looks to be only for lenses with a focal length greater than 50mm. When used with a wide-angle lens, the view through the viewfinder is one of extreme "tunnel-like" vignetting. There is some mirror cutoff in the very upper part of the frame when used with a normal lens or telephoto. The bottom of the screen is not a matte surface like other F-1 view screens, but is crystal clear (but it has a fresnel surface). It has a large, fine-edged microprism spot that extends out nearly to the edges of the metering rectangle. It is extremely bright! I use it with my 500mm F5.6 FL-F telephoto. I'm guessing Canon experimented with several different focusing screens for the F-1back in the mid-1970's. They either gave these to professional photographers for evaluation, or to Canon reps who would show people these screens at camera shows. Years later, some of these ended up on eBay, where I got mine. Too bad they never put the "Y" screen into production. I find it very useful.