That's a very interesting condition . The technical term for it is:
WORN OUT!
Insert bearings are built up in layers. The backing that contacts the connecting rod is made of steel, then there's a layer of copper (what you're seeing), then there's a layer of soft bearing material such as Babbit or other alloys, and then a VERY thin flash coating of pure tin, just about .0002" thick. It's there to allow the bearing to wear-in quickly to any imperfections in the surface of the crankshaft.
Finding bearings worn down into the copper as uniformly as those are indicates you've gotten all the mileage possible out of that engine without having a major catastrophic failure. Good timing on the teardown!
Jerry
