These models date back to the early days of N scale (late 60s / early 70s). They were originally tooled for MRC by Sekisui Kinzoku (aka Kato). However, the original tooling made use of some tooling owned by Con-Cor (for the core, roof, ends, etc) and said tooling was eventually turned over to Con-Cor under threat of litigation. The first Con-Cor production was actually in Japan. Later production was in the US.
The prototype for these cars is the PRR X58. Starting in 1964, the Pennsy started building these unique 50' outside braced, plug door box cars. The majority of parts were supplied by third party contractors: sides, doors and ends by Youngstown Steel, roof by Standard Railway Eq., cushion underframe components from Hydra-Cushion or Keystone and a variety of smaller parts by others. The Pennsy then assembled the "kits" on home built underframes in their Sam Rea shops in Hollidaysburg, PA. A total of 2565 X58 cars were eventually built by the PRR. In addition, 100 of these cars were built for the Lehigh Valley.
The X58 sub-classes (X58, X58A, X58B & X58C) are fairly similar externally. The main difference was the type of loader equipment inside. These loaders changed the car's light weight and internal dimensions considerably. This is a signature car for any late era Pennsy or Lehigh Valley modeler as well as all Penn Central and Conrail modelers. Many of these cars still can be seen across the country. Starting around 2003, a few have been spotted wearing full Norfolk Southern repaints.