That information comes straight from a man that worked the line at the Baltimore plant for 33 years starting in 1966. He was there and would think he would know about these things. I don’t believe the man makes up tales. He was very specific about this and witnessed it first hand.I find that very hard to believe. First off, their shouldn't be that many USABLE extra parts at the assembly plants once all the scheduled production has been completed, and anything further back in the supply chain could just go to parts distribution. As for NEVER using previous years parts in the next year's stuff, well, I have an all original 1973 454 from a C20 pickup with a steel crank. Every source I have ever found says that ALL 1973 454's got cast cranks. Where did that steel crank come from? Please don't read any of this to mean that I think the OP's car has the as built transmission in it.
As far as these parts not going back into the supply chain that was my first question when this GM worker relayed the information. He stated he does not know why they did it this way but someone at GM had a reason for this.