I promised OP that I would make an effort to track down the people who worked the activity in question and I took a considerable amount of time and effort to reach an informed conclusion.
Several workers were consulted including Key punch operators and floor supervision.
Everything started with the critical key punch action completed on a stack of IBM cards for each separate automobile produced.
Skip to 5:20 to get to basic information as to how GM used this technology to build cars in the late 1960's. Below is an actual IBM punch card used at GM Norwood.