@jase - you know that pic is a Mustang( albeit in RAAF livery which is odd) yeah? And the Merlin powered the Spitfire.
Not sure if your two posts were related, it's just how it read to me.
That is VH-BOB which IIRC a 1947 P51-D which runs the Rolls Royce Merlin G.
I believe earlier P51's (possibly P40's?) ran an Allison V12 but the lack of a supercharger limited the maximum altitude the planes could reach as a supercharger is reqiured in the thinner air, hence the switch to the Merlin.