from Warplanes Air Battles of WWI - Beekman House 1973 pg 32
On April 18, 1915, an event which was to have extreme importance in the history of aerial fighting took place. A well-placed rifle bullet fired by a rifleman named Schlenstedt, defending Courtrai railway station, fractured the petrol pipe on a Morane-Saulnier monoplane in which the well-known French pilot Roland Garros was attacking the line. Garros landed, but before he could set fire to his machine, it was captured by the Germans. The secret was then out: Garros who had destroyed several German aircraft in the previous few weeks, was found to have a machine gun able to fire forwards through the airscrew.
....The propellor, which was armoured with steel deflectors to avoid damage from the aircraft's own bullets, was shown to the Dutchman, Anthony Fokker, whose M5 monoplane was then undergoing service trials, and within 48 hours Fokker was claiming to have invented an interrupter gear to prevent bullets from hitting the screw. But then Fokker's brilliance as a demostration pilot was equalled only by his unscrupulousness and his flair for public relations. The irony in the situation lies in the fact that Fokker's new monoplane, which went into service as the E1 had been designed only after Fokker had aquired and analysed a Morane. Furthermore, Saulnier had himself invented and tried an interrupter gear but had discarded it because of unreliable performance of service ammunition. He saw steel deflector plates as an improvement and he may have been rigth in the early war years. 'Bad rounds' continued to bedevil machines with interrupter and sychronising gears on both sides throughout the war. This, rather than official stupidity, could possibly be the reason why the notion of using the airscrew to fire a gun had not been adopted before, though it is highly unlikely.