Pretty sure these are test vehicles, aka Manufacturing Validation Builds. GM historically builds 75 or so engineering test vehicles before the production line is ready, but using production tooling where possible. Then a few months before volume production they build a couple hundred MVBs on the actual production line. They have employees use these as daily drivers and report and manufacturing defects (and in recent years, s/w issues). GM uses this feedback to fine-tune production methods and their training methods, then starts building cars for customers.