This car was for sale in 1988 at P&P Performance in West Des Moines, Iowa.
Nice.
#2143 has a SAI completion date of 5/16/67. That seems late for the flat tail panel.
I did find a couple of other cars with the flat panel near that time, but the majority of cars with it appears to end in late April.
#2143 was for sale back (in August 2016?) at the Mustang Ranch in CO, shown with LeMans stripes added.