GMC had a 1st and 2nd series in 55. The first series was the "new design" and the second series the "blue chip". This is US GMC. I'm not sure how it was done in Canada.
Well that is a matter of some debate.
GMC did not call them 1st or 2nd series, though the model breaks occured the same as the Chevrolet.
I call my '55 GMC's 1st series to distinguish them from the Blue Chip trucks, but its a misnomer according to the GMC historians.