This is from an old post I found - "from my understanding in the '60's Chevrolet used 3 different manufactures for their frames. AO Smith, Budd, and Chevrolet. The OASmith and Budd were very similar but you can tell the difference from the top side by the shape of the weld where the front bumper support is fastened. One is in the shape of a V, the other is a straight line.The Chevrolet frame as it was refered to was basically a one piece tubular frame. I don't think 'canadian' frames were made for just canadian cars."
The poster is correct as the boxed referred to as Canadian frames were used in cars assembled at various assembly plants in the US too. Maybe it was simply a case of what frames were available from which manufacturer at a given time