As it turns out, the use of suffixes by BuAer and its airplane manufacturers during the war was not well regulated and more specifically, there was no published definition for suffixes C or D as there came to be over time for A, B, E, F (briefly and replaced in effect by Z), H (except for the PBJ-1H), N, P, R, and S. After the war, in January 1946, definitions were added for the suffixes J, K, L, Q and W.
In March 1946, however, BuAer decreed in Aviation Circular Letter 43-46 that henceforth—among other additions, deletions, and changes—that the suffix B would be used for "Special armament version" and C, "Carrier operating version of a non-carrier aircraft". In the case of the F4U-4C and the F8F-1C, the change was made retroactive so they became the better-known F4U-4B and F8F-1B.
This suffix history is described in full in a draft monograph apparently prepared by the history section at BuAer in May 1955. See http://www.alternatewars.com/SAC/Use_of_Suffix_Letters_in_Model_Designation_of_Naval_Aircraft_-_May_1955_Monograph.pdf