While I cannot help you with the "Stop" date for the production of 1973 Mustangs, I can attest to at least 41,900 being built after yours as I have a '73 Grandé #244824.
According to the Marti Report, it was built on May 29, 1973. Based on the numbers built for 1973, there should be nearly 10,000 more built after mine was built.
Keep in mind that the serial numbers begin with 100001.
Additionally, according to my source documents, the Mustang IIs were not released until the Fall of 1973.
Hope this helps!
i was going to say the same with the consecutive numbers its different between both plants.
i looked and you can't even find info on the very last 73 car produced, i would of thought it famous but no info exists
you would need the last numbers from both plants to figure out how far back your car is.
the only thing i found was August 73 they claim the 74s were introduced.
My mach 1 is 253468 and the fastback is 165947 both are 73's.
Do you have the date your Mach I (253468) was built?
Since that VIN is the @8644 unit (mustang) built after my Grandé which was built on May 29, 2010, it should give Steve (and us) a much better idea as to when the last one was built. By my math, there was @1000 mustangs built after yours.