It looks like the Powermatic transmission was a 6-speed automatic used on big trucks in 1959 (6000-10000 series).

On different size trucks, it was used a 6-cyl (261) or 8-cyl engines (maybe 248 &/or 283 &/or 322?).
http://chevy.oldcarmanualproject.com/chevyresto/59ted11.htm
http://chevy.oldcarmanualproject.com/chevyresto/59ted13.htm

Is there a colored ID tag on the transmission?
http://chevy.oldcarmanualproject.com/chevyresto/59tsn04.htm