Copper plated big end bolts

[Grumbly]

Something that has made me wonder, when stripping old engines (i'm taking about 60s & 70's here, i dont have much experience of newer engines) you often find some of the big end bolts have a copper coloured coating. This is presumably from the shell bearings backing material eroded by wear.

But what causes it to coat the bolts and why only some, whilst others remain black. I can't recall if it's always one bolt per rod, but this is the case on the current twin cam project engine.

Does anybody know, am i missing the blindingly obvious?

[Boosted LS1]

It's black ops, part of a covert chrome plating process.