5 June 1958
The "Organisation Européenne de Coopération Economique" (French) or in English "Organisation for European Economic Co-operation" (O.E.E.C.) ccame into being on 16 April 1948.
It emerged from the Marshall Plan and the Conference of Sixteen (Conference for European Economic Co-operation), which sought to establish a permanent organisation to continue work on a joint recovery programme and in particular to supervise the distribution of aid. The headquarters of the Organisation was in the Chateau de la Muette in Paris, France.
See also Official website of the Organisation for European Economic Co-operation. As you would quickly notice, the organisation still exists today... but its goals have been reviewed.
This organisation was quite unknown and philatelic material are the rarest to be found. Besides the few white covers with the cancellation, only three covers are known (R) and one typical maximum expo card (see below).
The Official Expo 58 cover had the special cachet as used for The European Council but with some adaptions. It was edited by Campo Rodan.
The next cover of the OECE has been edited by an unknown organisation, and is extremely rare to be found. The scan below is done by one of my "collector collegues" - thanks, Ken !
Another rare cover, also edited by an unknown printer
The OECE has also edited an official card during the Expo 58. (Backside with text still to come)
Large cover with the 6 belgian stamps, each cancelled individually.