O-ring for sealing the Dellorto VHB and PHF carburetor float bowl to the carburetor body (MG# 13933400): Dellorto part number 7451.00.30
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
The size of the O-ring (MG# 13933400) used to seal the Dellorto VHB and PHF carburetor float bowl to the carburetor body is 69.57 mm ID × 1.78 mm wide (UNI OR 2274). Viton O-rings are best, but Buna-N work just fine and are a lot cheaper. Make sure to purchase an O-ring that is sufficiently soft...something with a Shore Durometer (A Scale) = 70 is great.
Dan Eberhardt reports that McMaster-Carr part number 9464K391 will work well (Dash 039, Viton 75 durometer). Thanks to Dan Eberhardt for sending me this information in a private communication.