Maybe this nuance is well known but I was very struck when lotting something for ebay by a very unusual Mark One Livery variant. I found I was looking at pictures of a rake of GUVs in lined maroon with double arrows applied. The caption said in effect not to be repeated.
This was in an internal BR management magazine issued in 1967. Management Quarterly no 11 is the reference. If this interests anyone greatly you can follow it up in my robertatforsythe ebay auctions. No running numbers were given or visible.