5 Kings that ruled Greece

 George I, Constantine I, Alexander, George II, and Paul. I'm unsure of what exactly this stamp commemorates, perhaps the fact that George I began his reign 100 years prior to this stamps issuance which coincides with Paul becoming gravely ill and someone wanted Paul honored on a stamp and this 5 King tribute was subtler. Paul died in March of 1964.
 The story of coups, exiles and ascendancies is rather a soap opera. George I held reign for 50 years, which is the longest reign of any King in the history of Greece.

                

JimmyB