It does strike me as a historical thing. Not all nations have them. The resource I'm looking at lists them for us, the Kiwis, the poms, (all the same flash by the way although the poms are also depicted on Vulcans (its an old resource) with a bi-colour fin marking of just red and blue and a roundel of just red and blue!); no flash at all for the yanks, singaporeans, Koreans or phillipin Air Forces; with the TNI and the Malaysians having them.
All that said, a lot of the piccies I've seen recently of ADF aircraft (try the Defence Gallery) don't have it. I'm looking at one now of an F111, an F/A18 and a Hawk and only the Hawk has it. F111 is totally subdued roundels and relatively plain SQN markings on tail, 2OCU Hornet has red, white blue roundel and flash on tail with 2OCU colours, Hawk has subdued roundel but coloured 76(?) SQN markings. The P3C is totally subdued markings and no flash on tail as is the Caribou and the C130J that I'm looking at. All the VIP aircraft have it though as does the P3B but it's role isn't maritime patrol so no need for subdued markings.
Anyway, I've asked some people I know if they know the reason why it exists (or existed in the past). If they get back to me, I'll get back to you now that it's piqued my interest!
