It is possible that every 1965 mustang made was different. With the number of options they offered back then it is possible so the one of one does not carry much.
I have a 1972 Mach 1 T-5, German model, Q code, 4 speed, deluxe interior etc, etc, that got crashed back in the 70's. A front clip would fix it. It is the only one of these I have ever seen but not worth fixing so it sets and rusts outside. So numbers do not add value, or special does not add value. Peoples perception of the car adds value. You could get a Boss 429 mustang in 1974 for $1,000 due to no gas to burn. People did not want them they wanted gas mileage. The perception changed and now they are valuable. When those people die off and the perception changes with a new group of people the prices will drop again. Your grandfather valued his Model T highly but you probably have never even thought of wanting one.
David