Erich,
Thanks for checking into this. Here is a quote from Bob Mannel's Mustang and
Ford Small Block V8:
"The transition date between 5-bolt and 6-bolt Mustang 289 V8's was stated
in Ford master parts catalogs as August 17, 1964, or August 20, 1964,
depending on the catalog or part of the catalog. Examination of actual
Mustang serial numbers by Jim Smart (In Search of Mustangs) places the
actual date as August 3, 1964. As for 289 HiPo engines, a 6-bolt HiPo with
assembly date of August 4, 1964, has been found in a 1965 Mustang with an
August 10 1964 build date. So, the August 3, 1964, date appears good for all
versions of the 289. In this book, Mustangs with 1964 engines (5-bolt) are
considered 1964 or 19641/2 Mustangs."
It looks like all reliable sources point to about the same date.
Bob
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