OEM O-Ring used on the rear drive assembly on the following Moto Guzzi models:

V7 - All years, all versions

V85TT - All years, all versions

V9 - All years, all versions

Also used on the rear drive and transmission on the Breva and Nevada 750

Used on the Daytona and MGS-01 cam belt shaft 

Measures 18.77mm x 1.78mm

It is #4 on this parts diagram