That's not an '81 Mustang....it's an 87-89
Those seats aren't 4-eye seats. They are 87-89 seats. The fabric looks a little different, but the big headrests like that only appeared in 87-89 and '81 rear seat were not split like that at all.
These are the seats from an '82 Mexican Mustang. You can see the rear seat is not split and they used the small headrests
I think it's an 87+ Mustang.....87-89 specifically...with new vin tags applied. If that is a conversion, i'd like to shake whoever's hand that did it because it's factory perfect. EVery wire loom and wire is where the factory put it. I see nothing on that car from a 1981 model year Mustang...even residual brackets and wiring.
The underdash wiring is perfect 87+ wiring using E6 and E7 part numbers...again 87+ Mustang. The kicker is the looms all run where the factory placed them
The paint also looks original. It looks like the body was sprayed first and then the GFX were added just like the factory would do. Someone doing a conversion wouldn't do that.
An AWFUL lot of work to make an '81 look like a factory original 87-89 Mustang. I've seen people convert 79-86's to 87+ but you can always tell something from the original car is left behind...and NOBODY EVER goes through all the work to put factory stuff in.
This photo is what makes me think it's a VIN swap
http://img217.imagevenue.com/img.php?image=34490_IMAG5202_122_27lo.JPG
I'm starting to think the car was stolen from the US, imported to Mexico, swapped VIN's from a wrecked '81 Mexican Mustang and registered as an '81.
Look on the fenders, under the hatch and hood for additional vin stickers. See if there are any other VIN's on stickers here. Ford put the VIN number on every body panel those years in the form of a tiny sticker.