Yep, an alignment mark, used to make sure the transfers were lined up correctly when the stone was laid out. They usually got erased after they knew the plate was correct, but sometimes they didn't get all of it.
most T206s show small traces of them at the centers of at least one border. You can even spot them in different colors on the same card. And sometimes the erasing was a bit too aggressive and there's a border gap.
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