Hi thanks for your question. From your description these mark SOUND like British Hallmarks.
The crown in a square box followed by 18 again in a square box appears on 18ct gold in the UK. From 1798 - 1974.
The cat face, is the Leopard's head which has been the town mark for London since 1478.
The box behind the marks however is IMPORTANT. If the marks do not appear in the box they will be pseudomarks, RESEMBLING UK Marks.
Usually it's a profile of the monarch, indicating coronation or jubilee for example, on UK Sterling silver, Elizabeth II Golden Jubilee - ca 2002, Queen Elizabeth 11 face is in profile.
If you send in a photo of the mark it would help. You are welcome to upload a photo in a follow-up submission if you like. Please use "Cat Face on Rope Ring" in the subject heading so that I can quickly assign the photo to your question here.
About the "1/2 ring" line in your cat on a rope question, I'm not brave enough to ask how that came about ;-)
