Jane-DoH uses the term "crest" I think to describe the point of maximum thickness of the airfoil. In the P-51's case the max is at 45% chord - unusually aft for that day, when most airfoils had max thickness at 25 or 30% chord. And there was a distinct inflection at the 45% point so the thickness decreased rapidly as the air flowed aft.
I've been searching for a profile drawing of the NACA 45-100 to illustrate this - not successfully so far.