MarkB, just as HemiKiller Posted, if your vin starts with 2 then you have a 72 regardless of when the actual production date is. Production of the current model year vehicle normally winds down in July. Some vehicle lines may run a little over depending on product demand and scheduling. This gives the assembly plant time for maintenance and upgrades to the line to accommodate any vehicles changes and tooling.
Also keep in mind that with 5,000 + dealers and a new model show date usually in Sept, Ford needed time to produce enough vehicles in a short amount of time for it's Dealers to have a few of each vehicle in time for new vehicle launch date.
Also as Hemikiller posted, the days of carry over engines were all but gone for cars and light trucks. Ford had cut compression ratios drastically for 1972 to help meet new federal emissions guidelines. (Medium and heavy duty trucks were exempt). EGR valves and catalytic converters were on the horizon and made model year specific engines the norm rather than the exception.
The seat belt light you mentioned was another Government mandated requirement that was going into effect 1/1/72. Ford had everything already in place and started production installing the new belt warning system 12/1/71 which gave a little safety cushion in case there were any problems.
There was a seatbelt warning lamp for 71 and 72's up to 12/1/71 that was included with the $17.00 deluxe seat belt option. There was also a fuel filler cap change on the Mach1 after the initial launch. The Fed's were not happy because of some fuel leakage problems with the 71 style pop open cap during some accidents. No definite date on that so you will see some early 72 Mach 1's with the pop open style cap.
So you can see that a lot of the changes were subtle and were mostly in options and for the most part were not readily visible items.
