No, it doesn't. The third and fourth character designates which entity paid for the engineering. Once the part is designed and made it can go to any product in any division. The Cougar is covered in parts designed for several different Ford divisions including Mustang, Fairlane, and full sized Ford / Mercury. As is the case with Mustang.
I'm with Jim on this one. For the
engineering part number 4 character prefix, the first two are decade/year, the third is the vehicle line that the part was engineered for (there is an accounting aspect to this as you infer), and the fourth character represents the engineering division (which also is used in accounting/budgeting). The
service part number usurps the fourth character and replaces it with a Y (Mercury) or Z (Ford). There are always exceptions but this is the prevalent approach.