This is a bit of a puzzler, especially at first glance. The corner card for the U.S. Postal Service in Washington D.C. gives a clue, however.
This appears to be a House of Farnum unused FDC for one of the Official stamps issued in the late 1980's or early 1990's which was re-purposed by the the creator to obtain an event cancel for the Genoa NV July 4, 1991 Concert in the Park event. It does add a bit of pizzazz to the cover, albeit adding a good deal of potential confusion for other collectors later down the road.
I've found a similar cover on eBay, listed by bargainhuntersfriend, which is from House of Farnum, but in red and blue. These are the colors of the stamps and what I would have anticipated would be used for the FDC's. I've seen nothing with a green embossed cachet, so this may have been a color which was rejected by HF in favor of those used on the stamps themselves. This, of course, is just speculation. Perhaps someone else has one of these used for an official stamp showing the green color being used.
Given the stamps used, this was a philatelic creation by someone prior to obtaining the event cancel.
Hope this helps decipher the cover somewhat.