2 things. (#1) Yes, the RAF applied the serial codes to the underside of aircraft wings after the war. Likely for the same reason the USAAF did-for civilians to be able to ID planes being flown too low. Squadron and plane codes in Europe and the UK, last 6 of the serial number in the US. Later that was changed to the "buzz number" system (FF-*** for Mustangs) in the US and elsewhere. But (#2) what you see here is the RAF serial not the squadron code. The RAF had 450 Mustangs of various models that were listed with a "KH" prefix.
RAF Aircraft Serial Numbers Database What the photo doesn't show is the 3 digit remainder of the RAF serial number which was likely painted between the fuselage and the fuel tank/bomb mounting under the left wing.