Number 125 Dellorto 1486 series 5mm jet, used as a main jet on square slide VHB carburetors, standard on most US version big twin Guzzis from around 1970 until around 1980 including Ambassador, Eldorado, V7 Sport, 850T, T3, Convert, G5, SP1000, CX100. Many European version big twins used VHB carbs until the mid 80's, including 1969-1971 V750 Special, 850GT, California II, T4, T5, SP II. Also some US specification PHBH carburetors as used on V50 III, Monza, V65C, V65SP, V65TT and Lario models may use 5mm 1486 series main jets as well
Note: Some other VHB applications (Nuovo Falcone for example), some Guzzi PHBH carbs, as well as all PHF and PHM Dellorto carbs use 6mm 6413 series jet for the main jet function. For a 6mm 125 main jet for these carbs, see part number 6413125
Avoid costly and frustrating ordering errors, identify your carburetor and jet diameter before ordering jets.