This is a Leica 21mm viewfinder, made for Leica M cameras, but of course suitable for any camera with a 21mm lens and a hotshoe to clip it into. Also, 21mm on 35mm is practically the same as 47mm on 6x9, so suitable for that type of camera too.
It's a very nice, clear finder in decent condition. Looking for a quick sale at £200 in the UK, free postage, and I pay the PayPal fees. £10 shipping internationally.