My Marti report (posted here somewhere too) states a built date of july 6th 1973. I emailed Kevin Marti the following:
"There is one thing I'd like to ask though: It states a built date of July 6, 1973 and I was told that that was the very last day that the Mustang series I was produced.
I was wondering of you could confirm this?"
His reply:
"It was the last day Grandé's were built. There were a few other models of Mustangs built a few days later."
So there you go and yes, I possibly own the last Grandé ever build
(still need to ask him that)