Actually, the answer is simpler than you would think -
Canada Post issued too blasted many of them! I remember going to CAPEX 78 and people were buying them in quantity thinking they would be an investment in the future - which they turned out not to be. Also, it is not uncommon to have dealers still use these S/S on their business mail. They certainly are not scarce.
Hope this helps?
Chimo
Bujutsu