Again, beware. Check the door tag against the VIN stamped on the inner fender on the driver's side of the engine compartment, plus against the title to ensure they match. The door could have been replaced at some time and the warranty plate not changed. The VIN you have decodes as:
6=1966
T=Built at Metuchen NJ plant
09=2 door fastback
K=289 4V Hi-po engine
40422= consecutive unit number
If is a true K-code and has the original engine you have a true find if at a decent price for condition. It's worth looking at. Good luck.
Rich
1966 Mustang GT convertible