The dorsal fin you are refering to was an addition to the early D models to correct the loss of stability when the rear fuselage was cut down for the bubble canopy. Some B's and C's were retrofitted in the field with the dorsal fin. That's why you see some with the modification. BTW, the razorback B and C were faster than the D because of that. Apparently the B and C had less drag with the turtledeck.