...from the whatever it's worth department:

SF is where it was processed (same as an APO 583....from France 1945, but postmarked in NYC).

APO 500:

GHQ, US Armed Forces, Pacific
Australia ? Aug 42-7 Sept 44
Indonesia 8 Sept 44-2 Jan 45
Phillipines 3 Jan 45-1 Sept 45
Japan 2 Sept 45- 28 Feb 59

Forte, Jim and Richard W. Helbock. A Price Guide to US APO Cancels of the Second World War. (Lake Oswego, OR: La Posta Publications, 1996), 77.
MP