6 July 1958
Flanders was in 1958 integral part of Belgium. The Flanders is mostly known as the place were the official language used in Flemish. Flemish is in fact not a language, Dutch is. Both Dutch and Flemish are in written nearly identical, the main difference is in the accent used. You could compare it like the difference between English and American.
Flanders was represented as well as Walloon region was represented at Expo 59, hence the cancellations.
(still to be added) the official text of the speech given that day.
Only two covers are known from Flanders. Please contact me if you would have any others (with cachet of course).
Official Expo 58 cover edited by Campo Rodan
The Official edited cover with logo of Flanders (edition is not identified).
Cover with the cancellation of both Flanders and the Walloon Region