For the stamps of 2F and 4F Atomium stamps, a First Day Sheet has been issued. Up to now, I have only seen a a French version of this first day sheet, but it is 100% sure that also a Dutch, French and German version should exist. 

The First Day Sheet has only been issued for the 4F value (black print) on which many people put the real stamp on it, and cancelled them - see below.

It is officially known that no First Day sheet has been issued for the stamps 2,5F and 5F stamps. 

(additional information about the above First Day Sheets: besides the fact that the stamps are correctly represented, the cancellation is not correct: it is the cancellation of 17 April 1958 and not 17 April 1957 !)