Green Shield Stamps seemed to be everywhere during the 60s & 70s. The GSS scheme was an incentive scheme introduced into the UK in 1958. It was aimed at increasing custom at participating sales outlets. Stamps were given to shoppers with every purchase.
It just so happened that 33 was chose to be printed on all stamps. This had nothing to do with price or value of the stamps. Were a massive amount of people unwittingly participating in some sort of occult ritual?
The number of stamps each shopper received depended on how much was spent in the store. The more money you spent, the more stamps you got. The stamps were then stuck into a book.