Dutch Olympic stamps from 1928.
The Netherlands organised the Olympics about 75 years ago.
Financially, the Amsterdam Games were a disaster, because there happened to be a deficit of some 300 000 guilders. Without philately, this deficit would even have been 20 % larger because the eight Olympic stamps were excess postage stamps… The excess postage wasn’t mentioned anywhere, though.
The surcharges of 1 to 3 cent, which all in all was quite moderate, yielded some 63000 guilders!
On the stadium grounds, there was a temporary post office, the special stamp caught the eye through its unusual pentagonal shape. An official message said that all postal items thrown into the stadium postoffice boxes would be stamped with the special stamp.
This meant that the Olympic postmark was printed not only on the Olympic stamps, but also on the long-term series from that period (the Wilhelmina stamps). They are very rare!