No problem Dave. I just don't know if Lewis means early and late P-51D variants or the B and C versions The early D variant did not have the dorsal fillet, those were fitted after aircraft 44-13902 or retrofitted in the field. Some of the most photographed and thus well known D's did not have the fillet.
I know for me the Warbirdsim P-51's for FSX are a must have, as well as any P-51 A2A produces.