The Greek Social Security stamp is basically a receipt for Social Security contributions.
Greek workers were issued a Social Security book and the relative amount of stamps were stuck in the book next to the entry for the monthly contribution.
The system was in place until a few years ago, am not sure if its still applicable though
The photo below shows a typical example:

The first two lines show the applicable dates for the contribution
The third entry notes the amount of working days
The fourth entry notes salary earned during the period
The fifth entry notes Social Security contributions - equal to the stamps pasted on the relevant entry on the opposite page