Offices Abroad stamps are stamps that are issued for use in a foreign post office under a treaty with a foreign government. It allows the first country the right to maintain one of its own post offices on the territory of the second. Offices Abroad are usually the result of powerful commercial interests in the occupying country obtaining a treaty from a weakened central government and are seen primarily in the late nineteenth and early twentieth centuries in two main areas-the faltering Turkish Empire and China.

 Offices Abroad stamps for China were issued by France, Germany, Great Britain and the United State (these are the "K" numbers) and it was at Shanghai that the United States maintained its post office. Offices Abroad stamps are usually not avidly collected by the country that they were issued for. In the case of China, the treaties that established foreign post offices in China were foisted on a weakened Chinese state and the nationalistic pride of the current PRC collectors is little flattered by collecting these issues. Still, they are scarce. My rough back of the envelope calculation shows them rising in price tenfold if the current generation of Chinese collectors ever wished to add them to their collections. Since their prices now are little more than what a normal US back of the book set with this degree of scarcity should sell for, now would be a good time to add a set to your collection.