the switch to hex socket bar clamp bolt was before 1975.. iab's link lists it as around 1972..
(and it lists 1/a's introduction as c. 1960-3 ?)
i have stems from 1973/74 with the 6mm hex

giro d'italia and campione del mondo have their own logo to replace crest on one side

also, earlier (pre-1978 ?) bars i have seem to use a different grooving on the ferrule than later models;
3 seperate sections rather than one wide grooved area