Viewfinder Mask for SL66 with 4.5x6 and 4x4 cm film backs (in original box)
This is a viewfinder mask for the SL66 which overlays on the focusing screen frame and is locked down via the WLF / Prism pins. I'm not familiar with this one since at least on the box appears to designate also 4x4 frame lines - and I've never seen a 4x4 film back, but I guess probably there was one. This will be a rare piece. Looks to be in excellent condition and comes with the case and box.