Used: Rolleiflex 2x Tele-Extender for 6000 and Hy6 cameras
This is a light weight and useful accessory to have. It will double the effective focal length of the lens but still allow you to focus at the same minimum distance. This copy is in near mint condition and comes with both front and rear caps. It's the later version and will work with all cameras.
I have three of these - you could buy them all and stack them and get a whopping 6x! Just kidding, but I think I should try it someday.