This sale is for a mint condition and really rare Mamiya RB/RZ67 Bellows Lens Hood G-3 77mm Rotating Ring for the 100-200mm zoom. The instructions are (of course) in the Downloads section if you want to know more.
It is designed to keep your G-3 hood in the same place when you rotate the focusing ring on your zoom (both the RB67 and RZ67 lenses). It allows you to rotate the hood easily to keep it square on to the camera.
Very rare as it was always sold as an add-on to the zoom lens and never part of the standard package. If you use a G-3 hood with your zoom then you need one of these.