Today in Postal History

This registered airmail cover was sent from Brussels to Amsterdam.

It is franked with a complete set of the first Belgian
airmail stamps issued April 30 (Scott C1-C4).
Each shows a Fokker FVII/3m over a different Belgian city.
There are five Brussels bilingual CDS on the front with one
for each stamp and one in an open spot for legibility.

Registration was accomplished by adding a preprinted Registration
label for Brussels and a post office seal on the back flap.
The seal on the back is tied by two additional Brussels CDS.

When the cover arrived in Amsterdam an additional registry
serial number was stamped above the deep green 2 fr.

There is an intense blue bilingual airmail etiquette.

There are no transit or receiving marks.

This cover would probably be nice to have even though philatelic.
The complete set of airmails is delightful to behold.