To expand upon Peter's answer, this is from Scott's 2008 Specialized Catalog:
Quote:
U.S. Postal Agency in China
Postage stamps of the 1917-19 U.S. series (then current) were issued to the U.S. Postal Agency, Shanghai, China, surcharged at double the original value of the stamp.
K10 is the Scott number of disi123's example. The entire series is numbered K1 - K18.
Terry