Why is one 340 connecting rod bronze color?

Copper was used as protection against the flames when they rods were heat treated. You will see the same on many camshafts. Most were washed in a solution that removed the copper... some did not. No rhyme or reason...

There has been a lot of speculation and misinformation about this for years. Do some research on flame heat treating and you will see the same info.

20 years ago I often loaded GM engine parts that had been heat treated from a shop in in MI and I hauled them to the North Tonawanda plant in NY. I watched copper coated cranks enter the heat treat furnace and saw others go to the wash tank. Lunati had their cranks done there and I'd see racks of BBC cranks destined for heat treat.

It has zero to do with repairing undersize or mis machined parts.