Yes, indeed Middelburg, first half of the 18th century. Castle with date letter O for 1750.

If we use the book by J. de Bree, Zeeuws Zilver, the master mark, given the shape of the cartouche and the image in it, only one is eligible, Jacobus Kuyper. He was registered in Middelburg from 1731 and used two versions of a cooper's hammer as master mark. Below are two drawings of his mastermark.

Not a perfect match, we see some deviations, it could be the drawings, maybe they are not perfect or did Jacobus keep a third variant during his working period? It could also be that he used a damaged master mark.
Peter.