O-ring for sealing the carburetor enricher cable holder to the carburetor body (MG# 14938300): Dellorto part number 8297.00.30
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
On some Dellorto VHB carbs (certainly not all), the aluminum holders for the enricher cables have a groove to accept a small o-ring. The purpose of this O-ring is to prevent foreign material (e.g., rain) from entering the carburetor.
The size of the O-ring (MG# 14938300) used to seal the carburetor enricher jet to the float bowl is 8 mm ID × 1 mm wide. 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.
McMaster-Carr part number 9262K442 may work well. Thanks to Jon Sefcek for sending me this information in a private communication.
Dan Eberhardt reports that McMaster-Carr part number 9263K548 will work well (1 mm × 8 mm, Viton 75 durometer). Thanks to Dan Eberhardt for sending me this information in a private communication.