Bob, I am sure you are right that it was the wrong barrel. My reasoning, though, is that possibly, as batches of newly made, dated barrels came in to the assembly room, they are put in a bin on top of what is left of the last batch. Then put on a receiver. Older ones could be covered up, then eventually end up at the top of the pile months later, and finally put on a receiver.
I ran into a situation like this on a 3rd generation S&W semi-auto pistol . A couple years ago, I got one with the large, hooked trigger guard. When I contacted customer service for a BOD, the told me it was two years after the change to a smaller rounded trigger guard. I pursued it with them, and was told that sometimes one would be at the bottom of the pile when assembling.
Maybe after I pick it up, some good close-up pics of barrel and receiver will give a clue.