This is a 21mm viewfinder to clip into a hot/cold shoe. I used it for a 47mm lens on 6x9 film, which is pretty much the same as 21mm on 35mm film.
It's in very nice condition, all metal build, in the box with both end caps.
The viewing quality of this finder is decent, a lot better than the finder which came with my Fotoman, but if you're used to Zeiss finders, it'll probably leave you wanting.
It provides a nice big view though, and is so much better built than the plastic Voigtlander ones. Priced to sell at £90.