Wow, thanks for the photo SlamBam, the aircraft looks great with the new dorsal fin fillet! This new addition will definitely turn some heads, especially the die-hard warbird fanatics, since the original B model Old Crow didn't sport the later, required fillet, at least not while Bud flew it. I'm sure it makes all the difference in handling however! I've also seen some 'secret' photos of Old Crow sporting paper drop tanks from several months back. I'm looking forward to seeing it at Oshkosh this year, if it attends again. Some don't realize it, but the dorsal fin fillet for the B/C variants is actually quite different than the D model fillet. In the most obvious way, the B/C fillet is longer, going forward of the empennage joint, unlike the D fillet which stopped before the joint.
An interesting factoid that I discovered more recently, is that under the FAA, you cannot fly a P-51D without the fin fillet, as the T.O.'s which the FAA upholds, governs that all flyable P-51D's must sport the dorsal fillet, so there is really no chance at all to fly a D model restored as an early D-5 without the fin fillet. Somehow this is different with the B/C variants even though the same T.O. was given out to B/C's at the same time they were given to D models to fit the fin fillet during the war.
I know that Roush is very safety minded, hence the elaborate, modern cockpit on Old Crow, and I would bet that the fillet was added to increase the safety aspect, but also obviously the handling aspect as well.
