The first sentence is not entirely correct: the first 20 P-51Hs had tail surfaces about the same height as the Mustangs that preceded it.
The second sentence is true and worth everyone paying attention to.
The third sentence not so true: the taller vertical tail that was on the Cavalier Mustangs was NOT the first Mustang to which this tall tail was added. The Mustang in the photos below was flown with what some folks call a "Cavalier Tail" - but the photos clearly show a P-51B, test-flown by NACA in 1945. NACA had a P-51D (maybe two) with the same basic tall tail. The modification was rather "easy." They took of the normal tail cap and replaced it with a taller, 14" cap, which gave it the taller height. The later "Cavalier tail" was probably an exact copy of the ones of which I speak, a more than 10 yrs earlier than Cavalier even came into existence. FYI, the "strange-looking" cuffs on the wings were involved in NACA tests of some other aerodynamic device or whatEVER...