And look what arrived yesterday:
I ordered a reproduction of the original owner's license plate from these guys:
http://www.licenseplates.tv/
It turns out that the CWT-160 plate number that was listed on all of the car's dealership repair orders was in a unique 3-letter/3-number sequence that started in 1973 in Ontario. So it looks like that is as close as the original day-one plate as I am going to get.