From the P-51 Flight manual;
"A stall in the P-51 is comparatively mild. The airplane does not whip at the stall, but rolls slowly and has very little tendency to drop into a spin. You get ample warning of any type of stall. In a straight power-off stall you feel a slight elevator buffet about 3 to 4 mph above the stall
This however, from RAAF Publication No. 780 (September 1950) Pilot's Notes for Mustang. Page 32 Para 46 (b):
When no fuel is carried in the fuselage tank, slight tail buffeting occurs about two or three knots above the stall, at which one wing drops gently.
BUT...with fuselage tank full or half full, there is no buffeting to give warning of the approaching stall, but a series of stick reversals occurs just above stalling speed; At the stall, the right wing drops sharply, and unless immediate recovery action is taken, a spin may develop.....The aircraft sinks rapidly as stalling speed is approached...if the control column is held back at the stall, a wing will drop very rapidly and the aircraft will become inverted....power-on spins should never be intentionally performed....as many as five or six turns may be made after recovery action and as much as 9000-10,000ft may be lost.