Seen this before?

I saw this item on E-bay a few months ago, and the price was right, so it became a part of my collection. Actually, I pretty much got this for free, since the auction included a set of the overprinted souvenir sheets, which I later sold. I may even have made a small profit on the deal. Anyway, this is I believe a proof of the overprint used at the show for the souvenir sheets. I don't know much else about it. I don't recall seeing this listed anywhere. Anyone have an idea of how many exist? Since it was printed privately, it could be 1 or it could be thousands as far as I can tell.