I just spoke with Euclide (Joe) Lavoie about this. He is probably the only person left who can give any first hand information on this topic and he confirmed that they did put a two digit year in the stem numbers. Joe started at White in 1945 and it was being done on their wooden canoes from then until he left for Old Town in the 1960s. All of the company records were destroyed when H. E. Sargent's construction company purchased White so we don't have anything else to confirm this.
Benson