I note the running boards are the short ones as would come with a cab and chassis, not a pickup. It looks like they were that way to start with, not shortened after the fact.
My guess is one of three posibilities...
Sold as a flat bed dump and later, perhaps much later a pickup bed installed.
Or pickup originally and early or later it was made to a dump bed, running boards were changed.
Could have been ordered as a cab and chassis and assembled with a dump mechanism and a new pickup bed when about new.
Certainly there are other possibilities too. Coming as it is now from the factory new was not one of them. Could have left the dealer or upfitter as it is now though.
