It is NOT a USPS postal form. It is a US civilian censorship form.
The standard reference for this field is Broderick & Mayo "Civil Censorship in the United States During World War II"
This label is not listed there, but the Civil Censorship Study Group quarterly bulletin has listed many new labels.
The Examiner number "15004", shown on the label is from the Office of the New York Cable Censor. This makes sense given the return address.
The "1559" in the small rectangle with rounded corners is an Argentine censor mark.
Very nice cover.