French Ace Attacks Aviatik, Discovers Pistol CameraFound, no less, by the legendary Jean Navarre. Based available information, his discovery likely was made on 19 May 1916, when he shot down a German Aviatik C over Chattancourt, France. This event also earned him the singular status of becoming the first Allied ace credited with 10 victories.
(from Popular Science Monthly, September 1916):

And here's an example of a similar gun discovered by the British a year later when, evidently, it was still considered rare enough to be newsworthy.
(from the Washington Times, 19 November 1917):
