Now I've got to go examine my old worn-out chains with a magnifying glass! My assumptions has always been it would make no difference upside-down because of reasoning similar to above, the wear being internal in the links between the plates. But what if the links are worn more closer to one side and less in the middle? Turning the chain over would reverse that. If it even happens that way. What if each plate was bent infinitesimally in one direction because of a skewed chainline? Again that would be reversed. Is the chain actually worn that way, I don't know but hopefully if so there's some way to detect it.