War era civil reg. Lettering, always on wings as well as fuselage?

hello all

wondering about war-era civil registration lettering, what was the regulation regarding sizing & placement? Was it mandatory on the fuselage at a certain size, but optional on the wings? Or did it depend on the category or something similar?

Specifically I’m thinking (again) about Hawker’s Hurricane G-AFKX; as I’ve never seen photos of it clearly showing the wings, I can’t help but wonder if it had the reg. in large letters on the upper and lower surfaces, say similar to a de Havilland Comet?

Link to my ancient post on the same aircraft:

http://forum.keypublishing.co.uk/showthread.php?t=74537

thanks!

greg v.