Hi Cigogne,
Obviously you've not read my book "Early German Aces of WWI"

. While we don't have any of Leffers' combat reports which describe his combats in the Nie. 16 (that I'm aware of), on 3 September 1916, as a member of Jasta 1, Leffers shot down FE 2b 6934 of 2Lt F D H Sams and Cpl W Summers of No. 23 Sqn near Bapaume. The RFC crew survived as POWs, and after the war Sams reported that he had been shot down by a German flying a captured Nieuport.
Of course, you are aware that on 27 December 1916, Leffers was shot down and killed when he attacked FE 2b's from No. 11 Sqn. His probable victors were in FE 2b 7666, flown by Capt J B Quested and 2Lt H J H Dicksee. The RFC crew reported that the enemy was flying a captured Nieuport Scout in German markings.
Greg