Since the oldest boomers would have only been 27 the answer is ‘most of them.’ Then add in the fact that younger eligible voters are the least likely to vote. Then account for the fact that even fewer could have voted in the previous presidential election of 1972. No election at all in ‘73. So the boomer influence on politics in 1973 was close to complete insignificance.
I’d think the majority were born shortly after WWII; they had enough influence to end the Vietnam War in 1973 (or were those WWII vets protesting against that?).