Hello,
Yes, this spring (#72)is often missing. It does prevent any up and down movement of the distributor drive gear. Up and down movement of the gear would cause minor timing variations.
Caution, one should always be certain to push downward on the distributor after loosening it's cinch when adjusting the timing, otherwise this little spring will push the distributor upward disengaging it from it's drive gear!
Thanks for scanning in the nice diagram Jim!
Joe Alexander
Blacklick, Ohio