Here's part of an answer. If you look at the attached page of "Ford Car Parts" (aka MPC - it says "Ford Car Parts" on top of every page), 1975 edition, section 184, page 12 (available in this forums library), the first entry on that page shows the service replacement stock number for the front half of the heater box, the half where the motor attaches, as being a 1960 Falcon originally designed part - C0DZ-18A542-A, and the rear half as being a 1967 Mustang originally designed part - C7ZZ-18A543-A, the half with the "door". (Note: The heater door was changed in 1967 to clear the lower RH edge of the dash, but it will work on 64 thru 66.) As FOMOCO in a rectangle was an early logo, replaced by FOMOCO in an oval, it is possible to get a mix of logos - it's possible.
Jim