I think 450 was the smallest GMC that offered straight air. I don't think Chev offered straight air until the task force series. Straight air was a rare option in those days on a tractor because most trailers still had vacuum brakes and it was considered dangerous to have better brakes on the tractor than the trailer. And it wasn't until the mid 50's that a lot of people realized that straight air is much easier to maintain then vacuum over hydraulic.