O-ring for sealing the carburetor enricher jet to the float bowl (MG# 13936100): Dellorto part number 6173.00.30
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
The size of the O-ring (MG# 13936100) used to seal the carburetor enricher jet to the float bowl is 2.9 mm ID × 1.78 mm wide (UNI OR 2012). 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 9263K636 will work well (1.9 mm × 2.8 mm or JIS P3, Viton 75 durometer). Thanks to Dan Eberhardt for sending me this information in a private communication.