Gentlemen many thanks for these suggestions and to Graham Stuart via email: As suggested I believe there may be a possibility that the light was fitted 'last minute' in France, although A.C. were hardly inexperienced regarding the lighting requirements for road cars and LM5000 was driven out to le Mans, I wonder who got that job ? In any event all the lights offered have a requirement for a large mounting hole, yet the hole in the bodywork is only 3/8" and the present light, which has always been in place since 1965, has a spigot which fits this. So could a panel have been replaced and the original lamp replaced due to damage ? Seems unlikely as the original le Mans reflective patch, close by, is untouched.
This was not a one off light made by hand so sooner or later a twin will appear.....