Thanks for all your replies to my topic. I've learned a lot as always.
I know about the tube in the rear frame rail as one of the solid indicators that a car is the real deal GT. The car in question is a C code and has had a partial GT dress kit installed, but the rear frame rails have two small holes which what appears to have a tube inside. I have not had a chance to be with the car since we acquired it and check it out, I have a video and we will look for the shot of it and post it when I can.
The GT that we acquired last year had a larger hole with the tube but not two smaller ones, I use bent safety wire to confirm it had a tube in there. What's the deal with the tubes? Did non GT cars have tubes? I read that 1 or 2 percent percent of 65 and 66 cars we're GT cars. What increase in value does the GT package add to a A or K code Mustang?
Back to Jeff's suggestion to look at the date codes on the carlite glass.
One of our Mustangs, the original Carlite glass has a date code 6 weeks after the day the car was born! The glass may have come from a salvage yard or from Ford dealer inventory. The replacement could have been installed before it was sold if was sold as a result of transport damage or vandalism for example, or damage that occurred months or years later.
On close examination of the car and the side in which the late date glass is located, there appears to be damage that was repaired. The windshield molding isn't quite right in terms of fitment quality, and other small details which went unnoticed by myself.
The car was delivered new to the Seattle District, so my assumption based on all the available evidence is that a mean tree branch smacked somebody's baby. If I would have discovered this information before I purchased the car I would have still bought it, and it doesn't indicate that it's a bad car, but I would have looked further into frame damage which fortunately there doesn't appear to be any.
One of our other cars, a 66 vert, has 15K original miles and all the carlite is coded within 60 days of each other. That is something that I would expect in terms of date codes since Ford was on target to make a million of them before the introduction of the 67 models.
I also learned that rocker panels are the same part on all mustangs at least through late 1969. I think I remember seeing a rocker date code of 63 on one of our cars, and I have seen 65 on a 1969 Mustang.