I have a silver serving platter that has been in my family for years, probably since the 1920s. I think it is silver plate, but it is completely non-magnetic. It is monogramed with an ornate 'S', which is for my mothers' maiden name.

It has 3 marks on it. The first a pictorial mark. It appears to have a unicorn, a crown, and other things I cannot identify. Perhaps a lion on the left?

It has a mark that clearly says 9003 1/2 stroke 18. Would love to know what that means.

Finally, it has a XXXX stamp. Is that a grade of silver plate?

Thanks much for any help provided.