That distributor part number is shown in the 75 Ford Car Parts, section 120, page 5, page 23, section 120.4G, as being used on 1968/69 428 full size Fords and Police Cars (P/C) with a C6 AT and T/E. Is the distributor a factory installed item on Shelby Mustangs? It's possible. The book says no. The book says C8OF-D for 428 Q and R code M/T engines for 68 Mustangs (I assume that's your year). Comments should be addressed to "the book". Any documented variations will be happily incorporated in the Distributor Spreadsheet located in the Forum's Library.