Tim : With a C9AZ Part Number, and the D0DA Identification Number printed on the Speaker magnet itself, this can hardly be the "correct" Speaker. You are obviously looking at the 1965-72 MPC, which was printed in 1975. By 1975, many of the "correct" 1967 parts had been replaced by newer parts which "will work", but are not really the "correct" parts. This is just another example of why not to use the 1965-72 MPC for 1968 and earlier Mustangs. If you are looking for "correct parts" for your 67, I would suggest in investing in the 1960-68 MPC. Even in it, there are some parts that have previously been replaced by newer parts, but it is a lot more accurate for "correct parts" as it was printed in 1967 and 1968. In this case, the "correct" original speaker was Part Number C7AZ- 18808-C, and the Identification Number was C7ZA-18808-C.
That being said, and getting back to the "magnet question" (low profile round versus higher rectangular housed), the C9 speaker, as well as the two 68 documented speaker that Marty noted, might make one think that the change to the round, low profile magnet may have been one of the many later changes made in 1967. However, it does not explain the one Richard Posted of his early 67 with the round magnet ; unless the original rectangular one had been replaced before he got the car, or if it was just a Supplier variation. Although, John's 67, which IIRC was built after Richards, and both at San Jose, had its original speaker with the rectangular housed magnet.
Just my thoughts, but at this point, we don't really have anything different.
Bob