Everyone who does not have this MPC in their collection, should take advantage of this ; hey, the "price is right", what beats "Free" ?
However, as with all MPC's, one must not take them as "Gospel". In this case in particular, if you have a 1965-68 Mustang, this should be treated as a "companion" to a "more appropriate" MPC.
Here is what I mean by "more appropriate". In the early 80's, when my Ford Dealer Parts Department was switching from hard copy MPC's to microfiche, I was lucky enough to get from them both their hard copy 1965-72, and 1960-68 MPC's, along with the counter catalog rack that held them.
At that time, my only Mustang was my 1966. I started using the 1965-72 MPC, as I was more familiar with it, since that is what my Ford Parts Department always went to first (which makes sense since that would show the parts currently available). It didn't take long to find that there was a problem in using this as the "go to" copy. I found that many parts shown for 1966 carried C7 or C8 numbers, and sometimes even C9, with no indication of what the original Part Number might have been. A look at the printing date of the 1965-72 MPC will tell you why ; 1975. In 10 years, many, many parts had been replaced by a newer part that "would function" the same, yet be different than the original part. Of course, if you are not concerned with originality, this is not an issue.
Interestingly, I found this to be the case back in the late 60's when working on my 1957 Ford Retractable. I purchased the 1949-59 MPC a Text and an Illustration Volume) from my Ford Dealer ( which at that time was still available from the Dealer), and found a similar thing. Some 1957 parts had been replaced by B9, CO, or even C1 parts, A look at the printing date (1964) explained why. I then sought out to find an earlier version, and found that the only earlier one was a 1955-57 Chassis Parts, and a 1953-57 Body Parts version.
Remembering this, I went to the 1960-68 MPC that I had obtained from my Dealer, and solved a lot of problems. Even though it was printed in 1967 and 1968, there were still some parts that I came across that had part numbers later than C6's.
Since then, I have always used my 1960-68 MPC as my "go to" MPC for my 66 and then 67 Mustang, and then looked at the 1965-72 for any additional information that might have been added, and have relevance. For those with 1969-72 Mustangs, this is probably your best option since the next multi-year MPC printed covered 1973-1977 Models.
As I noted at the beginning, if you don't have the free 1965-72 MPC, by all means take advantage of this. I would also highly recommend, if you have a 1964-1/2 to 1968 Mustang, purchasing the 1960-68 MPC ( a Ford Licensed copy now available digitally for about $25). You won't regret it. Even though I have hard copies of all the MPC's from 1955-1972, I have digital copies of them also ; it's a lot easier to scroll through them on the computer (especially if they are book marked) than go to the castalog rack and flip through them.
Bob