Jim : I have a source/ documentation, which does not agree with the Nov 64 date mentioned.
While we all know that there are things in MPC's aren't always true, usually, when dates are given, particularly to the day, there was something to back this up.
That being said, the "date" appears to be 10/15/64, according to the 1960-68 MPC, this particular page being dated "June 1968".
Bob
Oops - your "date" applied to HP engines only. There is no date indicated for 260/289. There is even a "difference" between the 260 and 289 listing, 289 included resonator.
My interest in this comes from a car show pre-judging meeting I attended where the 64 1/2-66 team leader reviewed what we should be looking for. That was over 20 years ago. He wanted to make sure that all cars were judged to the same standards. The big issue was that if you found a discrepancy on the second car you judged, you had to ignore it if you did not inspect that item for judging that same item on the first car. It's called consistency. Each area was discussed, engine compartment, interior, trunk, body and undercarriage. I remember the exhaust (undercarriage) standards discussion. It included resonators, omitted on 6 cylinder engines, required on 8 cylinder engines, a general statement. You'll never guess who carried the sheet of cardboard to facilitate sliding under cars for undercarriage judging (I'm 6 ft tall, 240 pounds).
Jim