The "PWR" designation just means that the model is powered. Athearn also sold unpowered versions, known as dummys.
Athough GP-9's were built from 1954 to 1963, all that tells is the first year they would have been seen on the rails. GP-9's have continued to toil away since then. There are probably several hundred still in service today, although none with the Santa Fe, since that railroad is now part of the Burlington Northern Santa Fe.
Since it has flywheels but the older box, it's probably an engine built in the 70's to early 80's by Athearn. Altnough they tended to be a little noisy, a tuneup and some rewiring can make them into decent running engines. The details are not as good as modern engines, but they are tough as nails and will run almost forever.