The C model Mustangs were basically identical to the B's, except they were built by North American in Dallas. The dorsal fin extension was generally a squadron modification done in the field to aid in correcting the takeoff swing. The Aircam book #5 on the P-51A/B/C has a few color drawings of Bs and Cs with the fin extension.

Drop me an email and I'll scan them and send them to you.
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