[BomberBoy]

The official nomenclature was "DFF" - Dorsal Fin Fillet. It was available as kits for B/Cs and also for early D/Ks that didn't get it at the factory.

Here's a good picture of one showing how the shape is slightly different from the DFF used on the D and K models:
http://mm04.nasaimages.org/MediaManager/...rofileid=9