I have a 1965 mustang coupe but not sure why the VIN (or warranty number) stamped on the driver side has a 5th character that contains a "1" instead of an engine code.
It is possible it was a mistake at Ford . We had a 64 1/2 that was a D code motor code ,the fender apron showed a D code and so did the pink slip ,but the door tag had it as a C .we bought a new door tag with the correct D code