Greg,
Thanks for the education. I figured that must have been the case. I've only been lucky enough to document one pilot car, a '64 Impala SS. It did not have any features which were introduced mid-year - it was strictly a pre-production "test assembly", as most pilot cars are. It's body build date is 6D, way ahead of standard production which began around 8D. It actually has some '63 model year parts on it, such as carb, wiper motor, steering box and tach harness. It also has a couple prototype pieces of trim under the taillights that never made it into production. Neat stuff!
Unfortunately, we haven't dug deep enough in it yet to find a build sheet.
Your Chevelle is one rare puppy.
Verne