Fender tag typically has "Scheduled" build date.
The vehicle may or may not have been built on that exact day.
Reading an article on how the computer process was at the time,IBM cards,broadcast sheets,etc. a variance of days even weeks off was not uncommon.
May of 66 is hard to believe since the plant had to retool for the new for 67 body style.
But given the lower production of the Imps that were "carefully built" it is quite possible compared to the mass production of other Chrysler products.
Back then next year model production does not kick in til late August/early September.
Detmatt,being that early I recommend checking to see if she is a "Pilot" car for the Mobile Director package??
Who knows..maybe the 440 in it now is a warranty replacement motor when fairly new--again who knows??
Never say never with Ma Mopar,LOL!!