Bayonet adapter to use other Kowa eyepieces than those produced for the respective series of binoculars.
Screws into the eyepiece mount of the binoculars and thus reduces the diameter of the binoculars. Be sure to specify which tube model you have when ordering, so we are sure what you need. This ring converts so you can use Kowa eyepieces TE-14WD (30xW) and TE-9Z (20-60x) on your older Kowa TSN-820/820M (821/823 Prominar). Those tube models have now been discontinued and replaced by a single; TSN-82SV which is directly adapted for the above eyepieces.
Note that after conversion, you basically have to have special tools to restore to the original bayonet. We can help you with this in store, but it is not something you can do in the field to, for example, use several eyepieces.