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Looks like a plating bubble trapped between the brass plating and core.
The bubble looks quite solid and has a smaller dot on the bottom lobe. Its hard to tell in the picture, but there is something going on where the core meets the outer ring in that area as well. Looks like a slight gap and less that ideal fit between the core and ring.
It is not unusual to have a gap between the outer ring and the core. There are some out there that have a gap large enough that you can see through it. These usually bring a premium.