In Scott, under French Offices in Turkey (Levant), there is a note that: "nos. 29, 32-33 were used during the early part of 1907 in the French Offices at Harar and Diredawa, Ethiopia. Djibouti and Port Said stamps were also used."
1908 was when Ethiopia joined the UPU. Before then, they needed stamps from one of the coastal colonies for mail headed to the outside world.
Maury lists these stamps under Levant but describes them as "1906-08 - Emission spéciale des bureaux d'Ethiopie". My translation of the blurb: "Special issue from the offices in Ethiopia. These stamps were issued without surcharge by the three French offices in Ethiopia (The piastre was not valid). They were used in very small quantities until 1908. During the course of that year, these stamps were sold in the philatelic market. Collectors would send them to Ethiopia to be favor-canceled (ils reçurent des oblitérations de complaisance). The period of genuine cancels did not go beyond June 1908. Values for used stamps and on cover are for cancels on stamps that were actually used."
The stamps have Maury numbers 25-27, and Y and T numbers 24-27.
I recently found an example, postmarked Harar but date illegible:
All have "Levant" in the banner. There is a 25c blue of the Mouchon type (Rights of Man), the 50c Merson type pictured above, and a 1F Merson. There are color varieties for the Merson stamps issued in 1920, oddly well after the period they were used.
Does anyone know anything more about these stamps and their historical context?