Volker,
the 1.5 Strutters of No.3 Wing carried a square marking behind the fuselage cockade. The square would comprise a geometric pattern in red and white or blue and white. Each machine appearing to have a different pattern.
I can highly recommend a Bob Pearson feature on the OTF web site:
3 Wing article
Here you will find a couple of Bob's profiles and at least one photograph showing the marking that I've tried to describe. If you need any more let me know and I'll scan some shots for you.
Mike