The Swarovski 643936A is a replacement tethered push-on eyepiece cap identical to the one that was originally supplied with the Swarovski 8x56 & 15x56 SLC series binoculars (in a pair, of course). Replacing your lenscaps when you're not using the binoculars keeps dust and debris off the lenses and significantly increases the quality of your views.
Many fine quality binoculars come with push-on style front lenscaps that are easily lost-and a nuisance to maintain custody over in the field. This tethered-style objective lenscap, however, functions perfectly. It stays just out of the way yet is easily located when you're done glassing