I agree that it is a fake and in my opinion it is a poorly made fake. The two postmarks are carefully burned in such a way as to leave just enough of the top edge to show they are typical postmarks for most Hindenburg mail but they have all of their information that identifies which flight burned away. Secondly, the back side of the evelope is not burned inthe same way as the front part of the envelope. In the photo you can see the front and back edges do not match up in their burn patterns. When you suggest the stamps and denominations might be used to pinpoint the flight to the last flight this is wishful thinking. There were many flights before the last flight and they all used the same stamps and total postage cost. I would place this as an example of a fake last flight which would normally have zero value, but given the attempt by some collector a long time ago to unsuccessfully get the post office to help him perpetrate his forgery I think that some avid collector might be willing to pay a few dollars for it. If you are interested in Zepplin flight covers I highly recommend joining the American Air Mail Society as this is where such collectors are concentrated. The cost is only $30 a year and you get a monthly magazine, discounts on catalogs and comraderie with others who collect air mails of not only the US but all over the world. Here is their address
http://www.americanairmailsociety.org/