GT-350 - I wish. Got lots of friends that do (there's a whole bunch of money here in Silicon Valley). All I have are a 66 'K' car and a 65 calendar car (with the 'papers').
Jim

 

There is not a 100% sure-fire "cutoff"date...

However, FORD felt the changeover WAS significant enough that they did skip ahead several 1000 consecutive VIN's. The following method WILL 100% positively differentiate between an "early" (generator equipped) and "late" (alternator equipped) 65 Mustang:

At Dearborn (2nd digit plant code = F), the sequence numbers started at 100001 and ended 64-1/2 (generator) production at approxmiately 220000. When they started back up a week later on 65 production (alternator equipped), they jumped sequence numbers up to 250001. You will not find any 65 Dearborn cars (5F##*######) with a last 6 consecutive VIN between 220000 and 250000.

At San Jose (2nd digit plant code = R), the sequence numbers started at 100001 and ended 64-1/2 (generator) production at approxmiately 106000. When they started back up a week later on 65 production (alternator equipped), they jumped sequence numbers up to 125001. You will not find any 65 San Jose cars (5R##*######) with a last 6 consecutive VIN between 106000 and 125000.

Metuchen (2nd digit plant code = T) did not begin production until later in the year. There were no 64-1/2 Metuchen (Plant code T) cars.

It's the 250/125/0 rule...

 

Sure enough. The fifth diget in the VIN is enough to tell. There are no 'U', 'D' or 'F' code Mustangs built in 1965, all are considered 1964 1/2. If you want to argue 'K' cars in 1964, go to the Hi-Po Forum. I'll meet you there.
Jim