Why is one 340 connecting rod bronze color?

I have worked some metal to size and contour in my life.
A thin layer of copper may not "hammer out" when used to make finishing dimension.
A thick layer, even backed by steel, would muuush out quickly.

and when it comes to a press fit on a wrist pin, they're probably aiming for tenths (.0001) rather than thousandths. It's not like trying to bush a hole..