I agree with the others leaning toward a double ingot marble.
Here is a chart that shows when two single-ingots join together to form a double-ingot marble:
Basically, a 9/16" marble when doubled will be about 0.709-inches diameter.
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If you watch any of the Stephen Bahr videos that show marbles being made, you can see that double-ingots are quite common.
Just watch the guy with the tongs pluck out those oversized balls.
Back in the day, an operator had to operate multiple machines.
They didn't have time to sort, because they were busy shoveling and adjusting.
Those double ingots were culled out later during sorting and packaging.
Double ingots were disposed. That is why you see many from digs.
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Fun marble you have.
Might be a foundational piece for collection of "marble oddities".
Sincerely,
John