I'm not saying that cars didn't roll off the assembly line bored 0.020", and I'm sure that you have done more engines than I have but one thing that's for sure is that all of the engines that I have worked on all where a standard bore and I know this because of two reasons, first my dad always miked the bores and there was never any markings on the pistons. The only one that I was ever told that was bored by the factory was when GM supposedly bored the 396 to a 402. We used well known machine shops for all of the machine work that we needed done, now this was back in the mid '70's through the early '80's and none of them even turned a crankshaft 0.020" that I know of, at least they never did on any crankshafts that they did for us. Boring blocks 0.060" over, I have direct personal experience with that and it wasn't nothing but a over heating night mare.