Ford had many engineering design groups, one for each car line, and each had a letter assigned to it. That letter was used in the third position of the Ford part numbering system - C6AE. It signified which engineering group started the design and had design responsibility. Any group could borrow another's design, Mustangs used Falcon and full size Ford engines for many years. The first Mustang to have an engine with a "Z" in the part number was the Boss 302. This part crossover system extended into parts of engines, the distributor for 1965 289CID engine with a four barrel carburetor was from a Comet - C5GF-12127-A. Early carburetors were also borrowed, but the Mustang design group had to create their own as other cars had different requirements.
A few for reference: "A" = full size Ford; "B" = Fairlane; "G" = Comet; "D" = Falcon; "Z" = Mustang.
Jim