The Air Ministry and Ministry of Aircraft Production had regulations for everything. These are the authorised styles in the relevant Air Publication; "marking B" is for Day Fighters at home and abroard, 8" high with rounded numbers / letters, minimum of 6" from the tailplane leading edge. As painted in the factory but in service many (particularly in N. Africa, Malta and Italy) were re-marked smaller with re-paints. They should have been marked with original size and style but obviously not always followed.
The 2 subsequent Mk.Vc s BR113 (601 Sqn.) and BR114 (a special modification in Egypt) had correct style but smaller sizes. "C" was for Naval and amphibian aircraft, "D" for trainers. Colour was Night except for night bombers (Dull Red) and Coastal Command (Light Slate Grey).
Nick
raf-serials by
Nicholas King, on Flickr