I think you are correct in that some bodies built at the Rouge were sent to Canada for final assembly. They may have been sent to other plants in the U.S. The 1928 Coupes are unique in that most of them (or at least the early ones) have leading zeros before the production number. In fact, the only cars I had equated to the Buffalo, NY plant were 1928 Sport Coupes so the equation of BO for Buffalo is wrong. I think but cannot prove that these 1928 Coupes/bodies with the B,C and D (not BO, CO or DO) letter codes were assembled at the Rouge.
Tudortom would you be able to look at the 1928 Business Coupe to see if the second and third characters look the same? The number on my 1928 Special Coupe is D000313 and the second,third and fourth characters all look the same. This has caused me to think these Coupe cars or at least the bodies all came from the same assembly plant.
Dave