Walthers originally released these cars in 1998 (coinciding with their "Superior Paper" building kit). Walthers originally called them 60' cars, but have since started calling them 50' cars. Supposition is that they came up with the 60' figure by measuring the entire car, and later on decided to call them 50' cars based on the space between the bulkheads.
Here's what Walthers has to say about them -
Pulpwood is an important source of fiber for making pulp and paper. Logs of pulpwood are cut to uniform lengths and piled high on rail cars. With V-shaped decks and high bulkheads, these cars haul large, stable pulpwood log loads from the woods to the paper mill, where they are transformed into liquid pulp and rolled into paper.
Walthers ready-to-run Pulpwood Car is a southern style car constructed by SIECO (Southern Iron & Equipment Co.) beginning in the 1960s. It carries loads of wood cut about four feet long, and stacked into the V of the car, where each side holds the other in place. The model features a heavy die-cast body, styrene details, free-rolling trucks and standard couplers. They are available in three-packs ($26.98) or as singles with a fourth road number ($8.98), in six road names and undecorated.
These layout-ready models currently come equipped with Accumate(R) knuckle couplers (earlier versions came with Rapido-style couplers).