I have found why some reference numbers are printed on a red oval on some of the yellow boxes both British and French.
The red ovals can be found on the plain boxes and on the red / yellow boxes. Both these types of boxes are printed in red only, it would have been more expansive to print them in two colours.
examples : 066 - 069 - 80D - 150- -551 - 621 - 626 - 675 etc…
One more Dinky enigma solved.
I have found one exception, the 80 E 155 mm Howitzer and 641 Army 1 ton cargo truck have a plain yellow box printed in red and black and the ovals are black.
An other exception is the 33AN Simca Cargo Bailly for which the standard 33A box has been printed with red ovals with the reference 33AN.
Some yellow boxes, yellow or light yellow are printed with black ovals on the ends and red ovals on the illustrated sides. The reason for this can not be explained yet.
examples of yellow boxes 101 - 103 - 626 - 686 etc…
examples of lighter yellow boxes : 253 - 686 - etc …
There are also some late yellow Supertoys boxes with black ovals on the ends and a red oval on the lid.