This is a more complicated question, it is not just the Bolt standards markings, SAE is also just coming into play at that time.
My offhand guess would be that they would be SAE sizes, because the SAE sizes were developed based mostly on what the auto industry was already using, eg US medium weight hardware.
[cheat, multiply bolt shank (stated size) x 1.5 to get head size, only for SAE, only for moderate sizes]
so #2 (1/2x9/16) should measure .750 x .844