what is special about these stamps is the fact that the CDS had been printed immediately after the stamp design itself by the Dutch Stamp Printers Joh. Enschedé in Haarlem on a offset-litho printing press.
Stamp collectors - apart from the readers of the monthly Filatelie - probably hadn't realized what a great effort had been made to support their hobby nor do they realize now when they see these stamps that they are looking at something special!