O-ring for the fork top plug (MG# 90706222)
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Thanks to Charles Hamer for sending me this information. I concur with Charles that the 20 mm ID × 3 mm wide O-rings fit best. McMaster-Carr part number 9262K267 works great. In (mostly) Charles' own words...
I got some info concerning O-rings for the fork top plugs. It took me some time trying out a bunch of different sizes before finding a nice fit, the OEM O-ring comes straight from Italy but IMO does not provide a nice fit. It is almost unnecessary because it is so sloppy. For some reason it might have been intended by Guzzi engineers, but to me it does not make sense. So I found some good alternative O-rings in metric sizes, they can be found at Advance Auto Parts or McMaster-Carr.