Two Triumph 3Sws from the August 1940 RASC contract C7162 for 6000 machines but apparently curtailed at 3458 due to the bombing of Coventry in November 1940.
They display the "597" arm of service serial of No.21 Reserve Motor Transport Company, R.A.S.C.
I should perhaps have mentioned that the background colour would have been standard RASC red / green diagonal and that the white bar underneath duriing the early war period was used to indicate GHQ troops and command troops.
Most of the Home Forces Reserve M.T. Companies were allocated at command level, unfortortunately the National Archives indexing system gives no detail, so it would mean digging out the War Diary to have an idea where the location might have been.
My thoughts too, Mark. I asked on forum with a War Memorial sub section if anyone recognised it, but no response so far...no luck blagging a copy of 21 Reserve MT's war diary either, as that may give a clue.