According to ‘Above Ypres’, by Bernard Deneckere, on 10 May 1915, Leutnant Wilhelm Frankl, FFA 40, shot down a Voisin with a ‘carbine’, for which he received the Iron Cross First Class. I assume that he was a ‘Leutnant’ at the time, as Deneckere then goes on to say that Frankl was promoted to Vizefeldwebel later in the year !
This is the same Frankl who was awarded the Pour le Merite and commanded Jasta 4, and is the subject of the 1967 Nowarra booklet ‘The Jew with the Blue Max’ ?
Deneckere identified this as ‘the first German victory as observer’.
Another candidate for ‘Franz Kaestner’, Hauptmann Gustav Kastner-Kirdorf, replaced Siegert as OC of the BAO, until August, 1915, when he took part of the BAO to Metz to form the BAM.