We can only guess what the engineers were thinking back 70 odd years ago. The stress on the top stud would be much higher than the stress on the lower bolt. When you hit a pot hole, the weight of that heavy starter would tend to pull out the top stud; it needs to be strong. So not only NF threads but maybe stronger grade steel.
A similar situation situation exists with firewall mounted brake power booster and master cylinder. Hitting a pot hole tends to pull the top bolts out from the firewall. In the early 1980's GM recalled some pickup trucks; they added a strut from the master cylinder to the firewall. There had been some accidents where the brake assembly had pulled away from the firewall.
