I was thinking the same thing.  At least, I was thinking the same thing about the holes in the after part of the fuselage.  I don't know for sure about the P-51, but on other of its contemporaries, there was a hole aft, to allow ground crew to insert a bar and lift the tail for maintenance purposes, for example, when sighting guns, without having to load the aircraft into some kind of derrick rig to lift the tail.  Some USN aircraft had similar holes for a lift bar; the Boeing F4B series comes to mind.

Regarding the hole forward, if it's supposed to be there, it could have been a vent hole of some kind.  Here I really am in Speculationland, and I have to look at my photo resources, to see if the holes are there.

Don't know if that helps, or just muddies things for the time being.