Finnish Air Force fin flash from the 18th of March 1918.
On the 18th of march 1918 CIC of the Army general baron Carl Gustaf Emil Mannerheim issued an order Nr. 26 officially approving first Finnish Air force aircraft insignia.
All troops are now informed that our aircraft will use from now on as a recognition mark the following sign (swastika) in blue colour on the wings and fuselage, in addition a red flag with a yellow cross will be attached to the tail. 
The flag is known to have been applied on only two planes (F2 and F3). 

Keskinen, K., Stenman, K. &amp; Niska, K. (1996) Suomen ilmavoimien maalaukset ja merkinnat / Finnish Air Force camouflage and markings. Tampere: Apali (ISBN 9525026051)
Own work Sample of use on Albatros B.II
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