There's no such mapping, you could do a text search and if the item name matches then maybe US ASIN 2785334 should correspond to UK ASIN 3894948, or maybe some third party just named something ambiguously. They're essentially GUIDs and essentially separate businesses.

We all recognize this as an anti-pattern that has directly led to a poor customer experience, right?

Sorry this is just not true. ASINs can be the same cross marketplace. There are lower level identifiers available if needed