Fiber washer for sealing the bolt that secures the fuel inlet pipe to the carburetor body (MG# 13934700): Dellorto part number 4052.00.30
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
The size of the hard fiber washer (MG# 13934700) used to seal the bolt that secures the fuel inlet pipe to the carburetor body is approximately 6 mm ID × 10 mm OD × 1 mm thick.
Dan Eberhardt reports that McMaster-Carr part number 95601A310 will work well (ID = 1⁄4 inch and OD = 3⁄8 inch hard fiber washer). Thanks to Dan Eberhardt for sending me this information in a private communication.