I used to believe the above:
that the dorsal fin was added to the D model only, and it was because the fuselage was cut down. that's what all the books said.
However recent reading of some T.O.'s issued at the time show this may actually not be the case:
Several crash reports tell of P-51B's and C's crashing because the
horizontal stab was torn off during maneuvering. The report says:
"Unless a dorsal fin is installed on the P-51B, P-51C, and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a Snap Roll. To prevent recurrence the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"
A previous entry for another crash:
Sections II and III of T.O. 01-60J-18 had not been accomplished. The
stabilizer was approximately 20 percent below the strength of a completely reinforced stabilizer. It is believed that this type of failure will be completely eliminated after compliance with T.O 01-60J-18 and the installation of a Dorsal Fin and reverse rudder post tab."
A Supplement to Basic Technical Order (From old Hap himself) says:
"1. Due to horizontal stabilizer failures which are believed to have
resulted form slow rolls, all P-51B, P-51C and P-51D airplanes will not perform slow rolls pending the installation of dorsal fin and rudder reverse trim tab, and compliance with T.O. No. 01-60J-18."
Part of this T.O. 01-60J-18, it seems, was to "...use 1/4" rivets rather than 3/16" to attach the elevator outboard and rudder upper hinge fittings,....to stabilizer ribs, providing additional shear strength....."
Also, I guess that drilling out the rivet holes to take the larger rivets
didn't weaken the riveted pieces any - they must have had enough meat left over.
Also you can see photos of P-51B's or C's with the dorsal fin:
http://www.mustangsmustangs.net/p-51...tary/eto/6.jpg