These are also rare to find used and this one is in great condition and came in with a big lot of Rolleiflex TLR's. I think the reason you don't find many is the original cost of these was well over $1000 because of the huge prism inside. They do provide the very best focusing experience and allow you to hold the camera lower and closer to the body which is more stable.
The condition of this copy is excellent as you can see from the photos.
Fits all versions FX, FX-N, FW, FT and may also fit older TLR models that have removable waist level finders.
These will fit the 2.8F, 3.5F TLR's and are the best optical prism made for the camera. You will get 2.5x magnification and a full view of the screen. If you want the most accurate focusing, this is very helpful!