Concord977 From United States of America, joined Jan 2004, 1261 posts, RR: 22
Reply 1, posted (11 years 10 months 1 day 14 hours ago) and read 32767 times:
It is designed to snap onto the 110 adapter on your camera, as a replacement. Pull off the old adapter and slide this one into place. Its purpose is to seal out unwanted light from entering the camera.
Staffan From , joined Dec 1969, posts, RR:
Reply 4, posted (11 years 10 months 1 day 8 hours ago) and read 32767 times:
It's probably a cover to put over the viewfinder when it's not used, ie when taking long exposures at night with the camera on a tripod. Since your face isn't covering the viewfinder light can get in that way and reach the film while it's being exposed.