They were punch-outs, with glued backs to be licked and stuck. Supposedly they didn't work all that well, which is probably why they went with peel and stick stickers for the 1973 series and beyond.
Originally there were 44 titles, numbered on the backs. As time went on, due to cease and desist letters, several titles had to be pulled and replaced by duplicates of other titles. This led to there being different titles with the same number and different numbers with the same title, giving a total of 56 different die-cuts. Those 12 that replaced the ones from the original 44 that were pulled are called the number variations.
Notice that number 21 has two alternates, so apparenly Moron was pulled and replaced by Jolly Mean Giant, and then Jolly Mean was pulled and replaced by Maddie Boy.
The 30 stickers that were released in 1973 as the famous "1st series" were all originally released as die-cuts. There are 14 titles that did not make it into the 1973 1st series. These are known as the non-repeated titles.