Yes, Code "26A" on the Door Data Plate Denotes a "Deviation" which usually indicates the leftover '65 smooth vinyl inserts. Your seats do look original however and the covers look to be original Ford, though I have never seen that Shamrock marking from a vendor. What plant is your car from? What is the Scheduled Build Date on the tag? On the Bottom Front Side of the Bottom Original Seat buns in the crevas, There will be either a Black or Red Factory stamp dating. You need to see if that date on both bottom seat buns corresponds to the date 3-4 weeks that your car was built.. If not, Someone swapped all the seats in the car at some time.. That would not be uncommon at all...especially if a PO had multiple '66 mustangs or found better original upholstered seats somewhere else...I did this myself on quite a few car in the 1980's..(Yes, guilty..lol). If the dates do match within the scheduled build date of your car, The mystery will only get bigger, but you won't know for sure until you get to see your bottom seat buns on both seats..
Happy Investigating..
Tony Kovar
1966 Sprint 200 Registry
MCA#70001