This is a black line edge contraption for the Kostiner easil. You expose your print, then mount this such that the print is covered and the edges exposed in one direction; expose with white light. then remount the gadget so that the print edges on the sides can be done.
Why it's in my darkroom, I have no idea. All Saunders easels in here. If anyone wants it, it's yours for the price of postage. Seems to be new. Shipped from Toronto.