So I did a little digging, and if this is a 1958 then it is a 500-Series, most likely a 521 since only one of the rear axles is driven. And what makes this really cool is that is still works! Do you know what type of engine it has? Back then Kenworth used Cummins, Buda, Hall-Scott, & Waukesha if I'm not mistaken.
Just out of curiosity, what's the asking price?