Just before 3:30 p.m. Fordice was flying his P-51 Mustang, a propeller-driven World War II fighter jet
The last Mustang was built in 1945. The USAF changed from the P to F designation in 1948. So the P-51 was changed to F-51. Just like the P-47 (there were still some flying in Guard units) was changed to F-47Sad day.
Technically, this was an F-51. This just means it was built later. The Air Force changed from P (Pursuit) to F (Fighter) in 1947.