The Turkish Empire in the late 1800s, early 1900s must be a very interesting period for philatelists, with all the foreign post offices, various currencies, etcetera. Those foreign post offices were I suppose usually founded to facilitate trade. The Germans had had post offices in Turkey ever since 1870. In the early days, stamps from the "Home Country" were used, which can only be identified as having been used in the Turkish Empire by their cancellations. Later, from 1884, stamps were overprinted in the local currency, piastres and paras.
However, the rather unfortunate exchange rates were having a detrimental effect on trade. This held especially true for printed matter and samples. Many traders started sending these via the post offices of other countries. In order to try and regain this important source of income for the German Post Office, they issued a set in French francs, on 5 August 1908. This set was also to be the last set of the German post offices in the Turkish Empire, because the First World War changed everything and made the Turks close their country to all foreign post offices in 1914.
The stamps, by the way, are from the German "Germania" definitives, which I would say is my favourite German definitive set. One day I will have a great collection of these, but just don't hold your breath!