Interesting...it has the codes for a 50th, it looks like a 50th, however, as chris pointed out, the door panels (which apear to be original) are not right...adn there is no Custom or CST option package on that list either. (the chrome bumper and side trim options cought my eye on that)
Either way it is a pretty cool truck. Notice it has the dealer installed side step in the bed. An interesting feature, and an indication that it was bought to be a truck. (like most were back in the day) Original key too, that's pretty cool.