AG1 - Navigation/Position Lights
AG2 - Recognition Lights
AG3 - Landing Light
AG4 -
AG5 - Pylons (Drops Bombs/Tanks if added)
AG6 -
AG7 -
AG8 -

Wikipedia
The British Royal Air Force (RAF) was actually the first military force to use the P-51 Mustang. In 1940, the British ordered the aircraft from North American Aviation because they urgently needed a new fighter. The early Mustangs—called Mustang Mk I, Mk IA, and Mk II in RAF service—were used mainly for reconnaissance and low-level ground-attack missions, because their original Allison engines didn’t perform well at high altitude.

The RAF was also the first to test the Mustang with the Rolls-Royce Merlin engine, which transformed the aircraft into a fast, high-altitude fighter. This upgrade eventually led to the P-51B/C and P-51D, which the RAF also flew. In British service, these later versions were known as Mustang Mk III (B/C) and Mustang Mk IV (D).

While America used Mustangs heavily as long-range bomber escorts, the RAF mainly used them for fighter-bomber missions, reconnaissance, and supporting ground forces. They were used in Europe and the Mediterranean and played a key role in the final stages of WWII.