I have a '65 fastback that I can't figure out the vin#. I have a letter, to the previous owner, from the curator (at that time) of the Henry Ford Museum, and from Lois Eminger. Saying from what they can tell, the car doesn't exist due to the fact that the production numbers of '65 Mustangs didn't go that high. Just wondering if someone knows anywhere else I can go. Marti, has nothing on '65's. The vin # is: 5F09A601203  

I don't know but VIN 5F08A662011 is for sale at http://classic-collectible-cars.ebizautos.com/detail-1965-ford-mustang-2~door-3183542.html  

Is it possible it wasn't produced in the US? (I know at some point the were making mustangs in Mexcico)  

Yes, it was made in the US.  That is your warranty code number and not your complete VIN.  Here is how what you gave breaks down.  There were over a million 65 Mustangs produced, so the over 601203 number is perfectly within that range.  

Warranty Number: 5F09A601203
Year:  5  1965

Plant:  F  Dearborn, MI

Body Series: 09 2+2 Fastback

Engine:  A 289 4v V8

Unit:  601203  601203  

I thought the millionth Mustang was produced in March, 1966.  

I'm not going to look it up again, but certainly over 60 grand were made in the 65 production year.
OK, on edit:  There were over 607 thousand produced in 65, and 680 thou produced in 66, making the million happen in 66.  But, the important thing with regards to this thread, is that over 607 thousand Mustangs were made in the 1965 production year.  

The letter from Robert Casey, curator for the Henry Ford Museum says there were only 559,451 Mustangs built in '65. The car also has factory GT gauges, exhaust tips, & disc brakes, but it isn't a GT. I've had the car for 12 yrs, and so far I haven't found out anything. I guess it really doesn't matter, because it is now around 90% restored, and it is a ball to drive around in.  

Since the unit numbers started with 1000000, yours would be number 501,203 and well within the number produced.  If you are sure it wasn't originally a GT, someone might have put the GT equipment on it before you got it.  It may be original as there was no info in the VIN indicating a GT.  You have a correct engine code, instrument panel, brakes and exhaust for a GT.  

I had one too many zeros.  The numbers started with 100000.  

Just a quick note of thanks for all the replies. I may never know, but like I said, I'll keep the car. If I figure how to post a pic on here, I'll post it.  

Yeah, the last 1965 Mustang would have a number of 659,451 as it's unit number on the VIN.  You are well under that number.  There will be nothing on the VIN indicating it should be a GT.