I was reviewing my collection and found an old Airmail envelope. Scanned image of the same is attached. It was posted from USA to New Delhi (India) in the year 1964. Front side of this envelope is franked with \”FOUNDER\’S DAY\” slogan franking. On the back side, a label is affixed with the printing \”Post Office Department, Unites States of America. OFFICIALLY SEALED\”. What is this label ? Is any Philatelist have any idea of this type of label, please inform.