It is a Mustang Mk.I. It is not a P-51A.That's a -51A...I wonder if its not actually an A-36, or maybe a pre-cursor.
There were two variants of the Mustang Mk.I produced by NAA - NA-73, 320 aircraft with RAF Serials in AG series; NA-83, 300 aircraft with RAF Serials in the AL, AM & AP series. These are not P-51s as they pre-date the first variant of the NAA Mustang to carry the designation. These were bought directly and paid for by the UK Government. Mixed HMG and MG armament, moveable/variable front radiator intake ramp. Built for and used operationally in ETO by RAF.
Then there were 150 NA-91, which was the first to carry the P-51 designation as they were to be provided to the UK under Lend Lease. This was the P-51 (no suffix) Mustang in US service and the Mustang Mk.IA in RAF service with RAF Serials in the FD range. 92 delivered to RAF, 58 retained by USAAF. This was the variant with the 4 x 20mm cannon armament. Moveable/variable front radiator intake ramp.
Then the NA-97 A-36A Mustang. 500 ordered and built. HMG armament, first version with fixed radiator intake ramp, dive brakes and underwing hardpoints.
Then the NA-99 P-51A Mustang. 310 built. 50 delivered to RAF in part replacement for P-51/Mustang Mk.IA not delivered. Mustang Mk.II in RAF service, with RAF Serials in FR range. HMG armament, NO dive brakes, fixed radiator intake ramp and underwing hardpoints.