Cable adjuster lock nut for VHB, VHBZ, flat top PHF and PHBH carburetor throttle and choke cable adjusters. This nut has a ~6mm internal diameter. Guzzi part number 12937300.
Ambassador, V750 Special 1969-1971, Eldorado, 850GT, 850T, T3, T4, T5, V7 Sport, 750S, S3, Convert, G5, CX100, 1000SP, SP II, SP III, Strada, California II, some California III, Mille, some 1000S, Nuovo Falcone, V35, V50, V50 II, V50 III, Monza, V65C, V65SP, V65TT, Lario, Nevada carb.
No doubt fitting other Dellorto carb types, as it's used on numerous Ducati, Laverda, Aermacchi, Harley, Husqvarna and other brand motorcycles and scooters. Ducati part numbers 016027178 and 800004104