Just picked up this item yesteday at a flea market in Frontenac, FL, for $2.50. It appears to be a souvenir/CTO issued (by the DDR?) to celebrate the admittance of the DDR to the United Nations on 18 September 1973.
The stamp is SC 1492 and at least two are available on
ebay for $1.00-$1.90 for MNH.
Not really a cover, because it is unaddressed, but an interesting bit of history.
Donald