Four things for consideration:
1. Not all parts used on a Mustang were designed for the Mustang. This is what Dave is referring to. Although there is always room for exceptions, part numbers were generally assigned based on the first use case. If a part from a 62 Falcon worked for a 64 Mustang, it would be used, and continue to be used until there was a reason not to. I think that the ignition points are a B* (1950's) part number.
2. Parts designed for the Mustang in 1964 would have a C4 number. If there was no change needed for later years, the part would be classified as a "carry over". For those that work at Ford, there is no money budgeted to even think about a carry over part.
3. When the Cougar was introduced, there were plenty of Mustang parts used there.
4. When body parts were common for coupe, convertible, and fastback, they would get the coupe (65) designation, but used across the board.