Each period and nearly each squadron did have his own size and type of font of code for the airplanes... all this with rules of the command (air ministry) and sometimes when rules did change you did have the two type of font and size in the same squadron even some time two planes with the same indification letter (the third one) in one squadron (rare but it did happens). The serial number was unique but not always visible during the beginning of the battle of France the first squadrons of Hurricanes did have the serial number over painted. The markings of WW2 planes are a science and demand many hours of study.
