I have a 1966 Mustang GT Coupe with a trim code 66A. What does the "A" designation mean?

 

If it is behind the body style number, it means it had the standard interior (no pony seats). However, 66 is not a body code. 63 was the fastback, 65 was the coupe, and 72 was the convertible.

For example, our 65 convertible has a tag of 72B, because it has the deluxe interior (with the pony seats).

 

Thanks for your reply. The code I am referring to is the interior trim code which the charts show 66 as black luxury but mine says 66A.
Mike