This is what I got from one of my sources. He can't remember where his source was from so may not be true.
My understanding of the need for fin markings in WW1 was due to the mainplanes of a bi-plane depending on the angle viewed from obscured rondel markings on the fuselage. Also depending on the angle the aircraft was viewed from the roundels on the upper and lower wing surfaces may not be clearly visible.
The fin markings were adoted as red white and blue vertical stripes by the allies. The sequencing of the colours indicated the nationality of the aricraft but the vertical stripes made for a quick recognition of an allied aircraft.
Another example of how trigger happy some troops were, when the USA joined in to the war they initially had a roundel of a red circle on a white star on a blue circle. Because of identification problems and confusion of the star with the German cross, the USA dropped the white star in favour of a white circle until the end of the conflict.
The USA also had the allies fin markings added pretty quickly after joining the conflict!
I've also checked with a couple of former RAAFies at work and none of them had any ideas either!
