When Hershey started shipping chocolate bars around 1900, you didn't have refrigerated trucks; nor did many people know what chocolate tasted like as it was a luxury. So the flavour of what arrived after travelling in a warm truck became so popular that when Hershey started shipping in refrigerated trucks, the customers said 'this doesn't taste the way I remember!'. At this point Hershey opted for backwardly comptible flavours and began pre-oxidising the ingredients.
When it's metal, oxidising rusts it. When it's fat, it's a polite term for rancid. It's that far more than the cocoa beans used in the US that gives a Hershey bar its, well, 'distinctive' flavour ;-)