There are date codes on all the parts, cast, stamped so you can tell what day month and year they were produced.
The 73 has a different turn signal wire harness. The tail lights did not have the black out around the aluminum trim. There was a filler panel between the bumper and the tail light panel since the bumper was moved back.
There were rubber filler blocks in front of the bumper ends. There would be holes in the quarter panels to attach those fillers.
The round nylon stop blocks on the windows were riveted on a 73 bolted on 71 72.
The seat back release is to the outside of the seat back on 73.
Here is a link to see how the date codes are applied to the different parts on the car.
http://mustangtek.com/FordDecode.html
In order to put the front bumper and grill in for 72 he would have had to change the front fenders, bumper mounts, valance, etc.
If I remember correct Midlife, resident Ford wiring expert, said that the under dash 73 would not hook up to the under hood and headlight harness for the 72.
I would check the numbers under the fender.
The transmission also has VIN# on it. See location at the reference site for date codes.
If you had PW the bezels are different.
Build sheet was tossed into car on the assembly line and might still be under carpet or seats. Some come out great and some are totally destroyed.
There is also the Buck Tag that should be on the L.H. side of the radiator support tucked under the front fender. That has the VIN# and any items that needed to be put on the body before paint. Here is a pic of one in place and off the car. Also shot of where the VIN# is no the shock tower.
It would be pretty much impossible to change all the codes and parts to switch over a vehicle.
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