Fran Preve, an employee of the Tonawanda, NY engine plant for decades, was also an avid racer. In the mid 80's Chevrolet gave him access to the production records. He attended the 1986 National Chevelle Owners Assocaiation's Annual Convention in Peoria, IL. He had a seminar and covered many topics and then answered questions. I was there as our Northern Illinois Chevelle Club was the host club. I asked Fran about the mysterious 1971 SS LS-6 and 1971 SS 4-spd manual transmission Monte Carlo. He said per the Chevrolet production records no 1971 Monte SS454 was made with a 4-speed manual at the factory.
Based on his access to records I do believe GM can provide production information if they wanted. Just like Pontiac. He has never publicly explained the rare factory production cars as he does not want them to be replicated or "tribute" ones made. I was with him in Columbus, OH NCOA national convention in the early 90's when a shop owner came up to us while we were looking at Chevelles and said he had worked on a 1965 300 Deluxe Chevelle with a 396 and hydraulic lifters that looked original. Fran asked what color the car was and when the mechanic told him he said "go find that car". At the same time a Northern Illinois Chevelle Club member, Gary Swearingen, of Bloomington, IL said he had purchased a 1965 Z-16 but when it was delivered it had a 327 engine, but everything else was correct; dash clock, Tach, special rear trunk chrome, etc. He rejected it and an older woman eventually purchased it. When Fran knew the model and color of the 1965 Chevelle with the "big block" with hydraulic lifters he knew about these cars. A GM exec wanted the plain jane Chevelle with the IX code Z-16 engine (425hp 396 engine placed in the Corvette but they took out the mechanical lifter and used hydraulic).
A few years later my family spent some time with Mark Meekins, President of the NCOA, and his family in Greensboro, NC. He showed me pictures of the mysterious Chevelle that had the IX coded engine. It was outside Nashville. The brother of the GM exec who ordered it died and his brother got it. That explains why Gary's Z-16 was delivered with a 327. We subsequently found several other cars he knew were produced.