NL-3818553: Mathematical congress

Hello there! On 14/Jun/2017 I received a postcard from the Netherlands, sent by Nolleke. It shows a maxicard of the 10th mathematical congress in Vienna, Austria; it was issued in 1981.

Back of the card

Since the maxicard was some years old and not sent from Austria, she had to add some postage in order to reach this part of the world, so here are the stamps she used

But since this is a maxicard after all, here's the stamp on the front. It bears the stamp value: 4 schilling, which was the currency Austria used until 2002, when it was replaced by the euro.

 Cheers!