In that case it would have been a 12.5% compounded return, not as good as Buffett but still impressive for ~40 years of investing. Would have been only ~$20 millions by 1991 if he put it in S&P500.
I've heard this story many times, as I was a seasonal driver for UPS for a while way back when. The numbers apply to the corresponding dates, not annualized for inflation. He retired with 700k in 1952 dollars and that subsequently turned into 70 Million in 1991 dollars. His charity was legendary within the company as well.