Steve is correct about the "assembly line rejects" statement. This really pertained to when those cars were in production. For example body panels would be pitched into a bin or placed on skids to be sent to the main parts depo when any serious defect was found(such as a mis-aligned stamping/die cut-out) dent or kink from possibly being dropped and the like. From there, those along with a percentage of over runs would be shipped as the NOS replacement parts to dealerships as needed. This happened with everything to some extent, grills,lenses,assemblies, etc. I had back in the early 80's a pair of NOS 70 Cuda quarters where the side marker cutouts were off-set as compared to the recessed area for where the side marker light bezel would sit. It was definitely left for "the body man" to have to contend with, it could never have gone on a car the way it was.
We can even take the NOS conversation to bare metal NOS parts, compared to the factory primed parts...early grey primer to black and then red-oxide primers used over the years.
As for the lunchbox parts, Chrysler had a lot of smaller suppliers they would buy certain items from, they couldn't possible make everything for the cars and it was not economical to try. That being said, one or two suppliers could be making washers, nuts,bolts and the like one year, and another supplier with a lower bid could have the business the next year. Though they would have to keep with in the tolerances set by Chrysler, some operations to get the end result may have been altered slightly, finishes (if approved), markings, all if approved, but changes from the assembly line for previous builds.
All this is especially true due to the smaller size of Mopar compared to that of Ford and Chevy, where production costs were always more closely watched.
There really is no hard and fast rule for anything with Dodge,Chrysler,Plymouth which makes this hobby so interesting .