This is an official and quite ordinary US FDC, but the postmark is unofficial and very rare. A specialist retailer  noted: “There is no catalog which lists catalog values or prices for Unofficial First Day of Issue cancels. Unofficial cancels are generally fairly scarce on ANY stamp issue, as one had to be in attendance in the one city of issuance, and then go to some other location to secure the cancels ON THAT SAME DAY. So distance and time were needed. We are specialists in this area, and have a feel for approx value, and how often we have seen the cancels on items over the years. I must say i do not recall ever owning these NY World’s fair cancels on the Salvation Army issue, and so these also have two areas of collectors who might have interest in them.” (http://trentonstampandcoin.com/)