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Chert or flint nodules in limestone can often take forms that resemble fossils.
Ill second the fossil interpretation. The shape is pretty uncanny, and you have both the cast and the mold. It is possible that the chert deposited around the shell and then the shell was dissolved, allowing more chert to deposit. Explains why the cast and mold are both present. Id have to see a photomicrograph of the chert, but I would bet its a spiculite (formed from sponge spicules) and that it was buried in a peri-reef environment (possibly at the edges of a carbonate mound).
Reaching? For the cast and the mold to both be present, the shell would have to have been there during at least the beginnings of chert formation.
See if I ever agree with YOU again.