56 was the first year for tubeless tires on many trucks, and I understand that many buyers weren't excited about the idea and instead ordered the old familiar tube type tires. I know in my search for 22.5 wheels on 56-59 Chevy 1.5 tons most trucks I see actually have 20" wheels, even though 22.5 was supposed to be standard.
So 17" may still be original, buy optional?
17.5, 19.5, 22.5, and other xx.5 sizes are tubeless.
