The huge difference back then was the crowds fluctuated wildly. We could get 50,000 one game and yet 20,000 the next game.
We had 41 league games higher than 50,000 in the 50s (27 times) & 60s (14 times). I did a chart a while ago showing the whole post war attendances but I've cropped the 50s & 60s below. You can see the peaks but then the lows as the attendances were all over the place. I bet there was huge debate in the pubs back then about the empty seats and glory seekers
It could well be like the Atletico Madrid attendance like
@ErichZann posted
HERE but the huge difference is, there's no fans unable to get a tickets due to lack of supply or fans then having to pay higher match tickets. Do Atletico bring up the subject of empty seats and be bothered about it showing on TV as much as we do? I think it's as
@Blyth mentioned, a UK thing.
The truth is there's no perfect number for capacity and even now if we had a 63,000 capacity attendances would fluctuate. That could be due to opposition, league position but one of the main factors would be price. If we didn't sell enough then drop the ticket prices, especially for young bairns up to U22s. We'd soon find out the true limit of fans who want to go to the match.
The big difference being young fans growing up and going to the game and getting SAFC under their skin and becoming hooked

Also bear in mind, it's not all about match day revenue as due to having more fans being able to go to games, means those fans would be more likely to buy merchandise and other stuff that would mean more money for the club in some way.