The Flying Heritage Collection’s Mustang has a dorsal fin fillet added in in front of the vertical stabilizer.
This piece helps solidify yawing conditions at high speed that could cause failure of the horizontal stabilizer. Most sources say that this condition resulted from the “cutting down” of the fuselage seen in D-model Mustangs to incorporate the plane’s new bubble canopy.
However, the risky situation existed in the design even before the canopy modification. B- and C-model Mustangs had similar issues. They, too, were commonly retrofitted with a similar (but not the same) fillet at the front of their tails as the war progressed.