Origins of the fin flash

[JDK]

G'Day all,
We are all familiar with fin (or rudder) flashes on military aircraft, but I'm trying to find details and period evidence as to the rationale behind the fin flash as a specific marking. Most accounts just have "...and fin flashes" which doesn't help much.

I'm guessing that after the various 1914 experiments of Union Flags (Jacks) on British aircraft, the fin flash was adopted as a side-making matching the French system, that would work particularly with pusher types which did not have much (or any) fuselage keel area. However I'd like to see any references or proof to this or other origins.

Thanks in advance!