GOT IT
I found a booklet in the REME Museum archives today. It was dated February 1944 and referred to vehicles coming from North America to the UK. One paragraph is very relevant. It is for 'B' vehicles for which Ethylene Glycol has been authorised.
"In order to ensure that radiators which have been treated with Ethylene Glycol solution are easily recognised, it is essential that action be taken, immediately a cooling system has been filled, to mark the radiator. this marking will be uniform throughout the Army and will consist of a RED circle pained on the radiator header tank under the bonnet, or in an equivalent position. In cases where this marking may be difficult to distinguish under the bonnet, the circle will be superimposed on a white square. This mark will be painted out should the radiator be drained and not refilled with Ethylene Glycol solution."
Also, the booklet stated that Frost precautions are fully described in A.C.I 1475 of 1943. Has anyone here got a copy of this ACI as I can't find in the Archives.