Just to stir the mix, below is from June 1954 edition of the GMC Master Parts Book covering 1939 through 1954. The bad news is that is doesn't indicate what is "older model" and what is "later model".
It's a know fact that GMC trucks got the serial numbers stamped on the frame rail years ahead of Chevrolet. Chevrolet didn't start stamping the frames until mid year 1955 with the second series Task Force trucks.
Mike B
