Agree with Kantuckkean about the possible mounting - possibly to show support for the gov't in exile? And PetesPockets55 ideas about the possibility that the counterstamp could be connected to the Dutch Resistance are intriguing. A friend's mom passed away this year in her high 90's and her obit described her considerable service in the Dutch Resistance. Most people don't know anything about this Resistance - from the CIA library:
https://www.cia.gov/library/center-...tions/csi-studies/studies/spring98/Dutch.html
That counterstamp warrants far more research. Don't give up - it is not a random mark, IMHO.