That is interesting that the 1969 Ford Parts Catalog does not include the Mustang (nor apparently not the Thunderbird either). I never realized that, although my focus is mainly on 1955-1967. This is odd since the previous years all included the Mustangs and Thunderbirds. I have never come across a 1969 (or later ?) single year MPC for just Mustang or Thunderbird.
To get back to the question at hand for the worth of individual year Parts and Accessories Catalogs (or "Single Year MPC's" to which I have always referred to them as), I would go so far as to say that they are extremely useful, as long as you know what you are dealing with. To help explain this statement, a little history of MPC's might be in order.
I have been collecting literature that is applicable to my cars for almost 60 years now, concentrated the most on 1955 through 1967. This of course has included MPC's. In 1955, there was a single year Chassis Parts and Accessories Catalog, but for Body Parts, one had to use a Multi-Year (1953-55) Body Parts Catalog. For 1956, the Chassis Parts and Accessories Catalog was for 1955-56, and the corresponding Body Parts Catalog was 1953-56. For 1957, The Chassis Parts and Accessories Catalog was for 1955-57, and the Body Catalog was 1953-57. In all of the years, I have never come across, or heard of, an individual 1956 or 1957 MPC.
In 1958, Ford started printing individual year MPC's, with both Chassis Parts and Accessories, and Body Parts in the same volume. These continued each year, covering all models until (as we apparently just found out), 1969. From 1969 on, it is unclear to me, what all was printed in the single year versions. However, I do have a 1984 set that includes a Text Volume and an Illustrations Volume, and covers all Models, including the Lincoln Mercury Line.
Now here is the problem that exists with the single year MPC's.; note in the attached spreadsheet, the "Printing Dates". None of the printing dates cover even half of the Model Year's production. This means that any added parts, or change in part numbers after the printing date, are not listed. And don't expect to find them in the next year's "single year" catalog (unless of course that part was carried over to a Model in the next Production Year), as the single year catalogs only include the year listed ; they do not include prior year applications. Yet even with this constraint, the single year MPC gives the most original Service Part (which, by the way, may or may not be identical to the Assembly Line Part).
Ford's answer to this problem over the years, has been to publish multi-year Parts and Accessories Catalogs, called Master Parts Catalogs, commonly referred to as MPC's. These typically covered anywhere from 5 to 10 years, as shown in the spreadsheet. While this was a "Godsend" to the Parts Department, it can be a problem for those of us wanting to restore a car with parts as close to Factory as possible. Remember, Ford didn't print the MPC's for people wanting to restore cars 50 or more years in the future, but for the purpose of maintaining or repairing cars to keep them functional and safe. Thus, over the years, many parts were improved, or changed to be used on other vehicles, but still being able to fit and function the same as earlier parts. Many times, these "improved parts" look nothing like the original Factory part. With this in mind, it only makes sense that the farther removed from the first year covered by a set of MPC's, plus the date of printing of the MPC, results in more probabilities that a part has been replaced by a newer part, sometimes several times.
Therefore, in my opinion, the best option of finding a Service Part as close to the Factory part as possible, is to pick an MPC in which the last years covered are closest to the year car you are working on, as well as the printing of the MPC also being close to the year of the car you are working on. To illustrate this, I would pick the 1960-68 MPC for 1964-1/2 to 1968 Mustangs ; the 1965-72 MPC for 1969-1972 Mustangs ; and the 1973-1977 MPC for 1973-77 Mustangs.
In addition, I would want the single year MPC for the car that I am working on. This is the ideal situation ; the appropriate Multi-year MPC to be used in conjunction with the appropriate single year MPC.
Just a word to the wise ; typically, the single year MPC's are short on illustrations. Many times an illustration for only one Model is shown, it being "typical" for the other Models. This changed sometime in the 80's when Ford went to the single year MPC's having a volume for text and one for illustrations. While part of the reason was due to the inclusion of the Lincoln-Mercury line and additional Models being added to the Ford line, it also included many more model-specific illustrations.
I hope this explains a little more about MPC's, particularly the single year versus Multi-year.
Bob