There was only official first day cover, format like a normal envelope (not a first day cover format) in bluish background:

same cover with perforated and imperforated stamps

Alternative with block of 4 of each issued stamp in perforated and imperforated version:

The only other private cover which can be found was issued by the Philatelic Club de Belgique - no info about number issued

Maximum cards are easy to find but not with the day of issue so not easy to find:

There was also a postal card issued with the same design as the first day cover:

unused version and used version with first day cancellation: