Like nothing I've seen either. At first I thought that might be a shell, but the reverse appears to be the same and I couldn't find images with double shells. I think you've stumped the board!
There is a part of me that wonders if it might be a medieval pilgrim’s badge. Many of them were in the shape of a scallop shell. I didn’t find anything online quite like this one though
Maybe something like this: https://pro.coinarchives.com/a/lotviewer.php?LotID=389918&AucID=705&Lot=318 Phil Davis
I'm with pilgrim's badge/token. I don't know where you do your MD-ing, but I get a Camino de Santiago de Compostela vibe from this one...
I have seen a few double shell coins, including this cast one. This does not ID the coin though. Anonymous, Rome, c. 230-226 BC. Æ Cast Sextans (32mm, 33.64g, 12h). Shell convex; club below. R/ Shell concave; club below. Vecchi, ICC 62; Crawford 27/9; HNItaly 322. Green patina, Good VF