I wouldn't say they are "rare", but if it has the fleetside box, they only made that style 2 years (58-59). Many options (like other years) make the truck more desirable such as big rear windows, delux trim, V8, etc. The body style doesn't seem as desired as the 55-57 style front end, but those of use who have the 58-59's seem to love them just as much.
I would think that the 31 series (short box) especially in a Fleetside, is more rare.
