Canon Magnifier S in excellent condition.

This attaches to the viewfinder of Canon cameras to provide a 2.5x magnification of the image centre for critical focusing (e.g. during close-up photography) or can be flipped up to provide a normal view. See the illustration here:

http://www.bobatkins.com/photography/reviews/canon_eyepiece_magnifiers.html

There is a dioptre adjustment to tailor it to your own eyesight.