My 71 Challenger R/T with a SPD of April 24 1971 has a 000DNU blue and yellow plate.
Taking a educated guess, that car was probably sold May/June/July 1971. It was not special ordered.
I think that car was in Los Angeles all it's life. I've heard their DMV offices went through plates faster than less populated areas. So less populated areas might issue earlier/lower plate numbers at the same point in time due to more lingering plates in stock.