Covers from the First Meeting of the Vietnam General Assembly in 1946 are being offered on eBay by a French seller (click here). The interesting thing to me is that he has a stack of these things.
Also offered are unaddressed covers with the Viet Minh cathedral stamps affixed and canceled, as well covers with the 1953 Telecommunications Festival stamps with red cachet (Richard Aspnes lists the cachet in both red and blue on his Commemorative Markings CD). Again, small stacks of these covers are shown, but only a single set seems to be offered.
I'm not planning on bidding, but I contacted the seller out of curiosity to see whether he accepts PayPal and ships to the United States. His reply, "Hello, bonjour. I don't accept paypal, but you can pay me with dollars and shit off paper in envelope - merci. BRUNO."
If you've been looking to pick up examples of these covers, the opening bid price is right (9 euros) although dealing with the seller might be interesting. Let me know how it goes.