Rocker arm clearance aduster lock nut. This is an 8mm fine thread nut. Big twin from 1967 through roughly 1979, on these models: V700, Ambassador, V750 Special, Eldorado, 850GT, 850T, T3, V7 Sport, 750S, S3, 850 LeMans, CX100, LeMans II, SP1000, Convert, G5. Then superseded to smaller and lighter lock nut 10034100.
This nut is also used on the adjusting screw on the actuating lever at the rear of the transmission on all 4 and speed transmissions from 1967 through 2002: V700, Ambassador, V750 Special, 850GT, Eldorado, V7 Sport, 750S, S3, 850T, T3, T4, Convert, G5, SP1000, 850 LeMans, LeMans II, CX100, SP II, SP III, Strada, LeMans III, LeMans 1000 all years LeMans IV and LeMans V, 1000S, Mille, California II, California III, California 1100, Quota, Quota ES1100, Daytona 1000, Centauro, Daytona RS, 1100 Sport carbed, 1100 Sport injected, EV and EV Touring 1997-2002, Jackal, Bassa, California Special, California Stone 2001-2002, Special Sport 2001-2002, V35, V50, V50 II, V50 III, Monza, V65C, V65SP, V65TT, Lario, Nevada, Nevada Club.
And it is also used to secure the sidestand cut out switch on: California III, Nevada and Nevada Club, 1100 Sport injected, Centauro, Daytona RS, EV, Jackal, Bassa, California Special, V11Sport 1999-2001.