Cat. No. 24x36 mask-new-old-stock
This is an overlay that fits over (carefully) the focus screen in all 6000, SLX, SL66, Hy6 cameras. It masks the finder to the smaller 24x36 crop. I actually don't know of any solutions to use 135 small format film in the Rollei SLR's so possibly for use with digital backs? New old stock.