I have never seen a 65 without seat tracks for both 5th and 6th seats. But we did work on one aircraft (I'm not sure which year, but it was pre 1973) which did not have the pilots emergency exit window (LH side window).

If the aircraft does not have the emergency exit window, then it is only legal for 5 seats. Same reason why all pressurized models are legal for only 5 seats. (exits have to be on opposite sides of each other so baggage door doesn't count.)

Check your aircraft to see if you have the emergency exit, If you do, then installing the seat rails and seats can easily be done on a 337 form using the parts manual as reference.

If you don't have the emergency exit, it can also be added without too much trouble.

Ray Torres