The two shields on the reverse identify this as a 6 Gröscher, instead of 1/6 Thaler, although that's usually immediately apparent by the size and weight as well! In 1756 Brandenburg was using the Reichsthaler (of 24 groschen / 288 pfennige.)
1756, Brandenburg-Prussia. Olding 359a, Cleves mint.
That being said, it is badly underweight and also at least 1-2mm undersize. In addition, the C cartouche (for Cleves) at the center lower reverse is missing its bottom half, probably due to a slightly off center strike to the south.
I'd be suspicious of a contemporary counterfeit, or a real coin with loss of metal (maybe saltwater immersion and dissolution with those surfaces.) Overall the surfaces do not look "right." Very grainy and porous.
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