Not too sure, but it IS in the POH. Also, the Mustang has a really nasty developed spin. It can eat up 10,000 feet recovering! If the pilot ctaches the spin in the first turn, recovery is almost immediate. When it develops, the nose oscillates from almost vertically downward to just about slightly nose up, and it can take more than a few turns to get the recovery timed so it works.
Fortunately, the P-51 is well mannered and gives the pilot a good warning before it stalls. If you pay attention you can avoid the stall-spin easily. If you are pulling some g's and feel the stall warning, you'd best not pull any harder and would be well advised to back off the back pressure slightly for saftey. Most really good flying planes gave a good stall warning.
The Fw 190 was famous for having almost no stall warning at all. It is an example of a good-flying plane that was difficult to fly near the limit due to the lack of stall warning. Maybe the experts could do it, but more than a few stalled and went in at low altitude in a vertical reverse.
The Bf 109 had no such issue and gave a very pronounced warning near the stall. You could horse it around right at the edge of stall and be comfortable doing it. In fact, you could still point the nose even IN a stall. That was one of the reasons why the top scoring aces of the Luftwaffe stayed with the Bf 109 ... they were very familiar with its stall characteristics and were not impressed by the Fw 190's stall at all. Familiarity lends comfort in a tight situation.