The dealers that sold big rigs had a lot of leeway as to how they ordered and sold their trucks. Buyers could choose their preference for specific types of equipment, within reason. Is there a specification sheet (sometimes a metal tag) on the inside of the glove box door? If so, it usually lists the engine, trans, and rear axle options the factory installed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!