Australia - 1940s War Department (Air Corps) censored cover to USA with Postage Paid postmark (no date, probably on purchase).
Note "Penalty for private use to avoid payment of postage, $200" at top right.
Paid 3½d postage rate with KGVI 2½d and QV 1d so cover should be sent after 10 Dec 1941.
Cover neatly open at back and in good condition.