Valve or rocker arm adjustment screw for early push rods as found on V700, Ambassador, Eldorado, V750 Special, 850GT, V7 Sport, 750S, and some early 850T.
These screws have the round head profile to match the early push rods with the "trailer ball" profile, and will not fit the later push rods with the blunt rounded end. The threads on these hardened screws are easily broken by excessive torque when setting valve clearance. This screw is also used on the transmission arm on some models to adjust the clutch actuating arm.
Most later big twin Guzzi models use a different valve adjustment screw, see part number 17034050.