This seems to be postmarked 1940, but the stamp was issued in 1945, It talks about National Air Mail Week - those were held from 1938 to 1940 plus I have not found a reference to this USPS and Coast Guard helicopter mail cachet. Any thoughts?
It looks like a 1946 cancel to me. Notice how the circular part of the last digit is only two-thirds the height of the other three digits. And I can see a trace of the hook at the top of the digit that makes it a 6 rather than a 0. It is just a poorly inked cancel with the hook at the top of the 6 being almost un-inked.