[skil55voy]

The observations are well taken. Part of the problem could be the distribution channels at the time. I grew up in Detroit. In the 60's the first series of a particular year usually showed up in April. Then of course the next series was staggered and so on. As a kid from packs I had almost a complete set of 61's. The 7th Series made it to my stores in late August. I had a full set of 62's. All series were relatively available. In 63 I never saw the 4th series. In 64 I never saw the 5th series. In 65 I had a full set. My friends and I were in our neighborhood market when the 7th Series arrived. In 66 and 67 no 7th Series.
In 68 I was again able to complete my set. I ordered the later series from Topps in 70 and 71. The wholesalers were the ones. The guy who owned my neighborhood market would drive to the wholesalers and pick up his stock. I remember him saying in 66 that when he went in late August all he could bring back was football cards. The 63 set was particularly dry. Never saw cards after July.