2 Different Metals Stamped Together?

I think your first hunch is correct, something has stripped the oxidation off the copper surface leaving one area lighter than the other. It didn't happen too recently though since the lighter area is already mostly retoned. Measuring the size doesn't prove much because the areas are quite similar in size and whatever stripped the oxidation would not necessarily have to act in a symmetrical manner. Improper mixing of the copper alloy takes on a completely different look- https://goccf.com/t/43592&SearchTerms=woodgrain