This sale is for a near mint condition 77mm adapter ring to fit the Mamiya RB67/ RZ67 bellows lens hood G-3. This allows you to fit your G-3 hood to most of the Mamiya RB67 / RZ67 standard and telephoto lenses using a 77mm filter ring – or for a host of other lenses as well.
This ring has only minimal signs of use. Try finding another one! I guess it will also fit the more common G-2 hood but I have not tried it.