Rear drive ring nuts
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Rings nuts are also commonly referred to as lock nuts, spanner nuts, prong nuts, retaining nuts, and lock rings.
Thanks to Tom Halchuk for sending me information on sourcing these parts.
On the V700 and Ambassador rear drives, a ring nut was used to secure the pinion bearings in place. Although it is easiest to simply upgrade to a hex nut, as is used on the later Eldorado rear drives, a ring nut can be used. The Whittet-Higgins Company manufactures ring nuts and special locking washers of the proper dimensions.