Cat. No. Used-Rolleimeter
This is a rangefinder focusing accessory for 2.8F (and I think also 3.5F) Rolleiflex TLR's. In very nice condition. Comes with original leather case.
I found a nice picture of what this looks like mounted on a flickr page by Carlos F. Rolleimeter and Rollei Pistol Grip | Flickr - Photo Sharing!