it could be an antique diamond ring. The hallmarking system in the UK in earlier times was a little erratic. It was felt that the value of the stones was greater than the value of the metal!
PLAT for Platinum is an expensive, hard & dense white metal with a greyish caste, often used around diamond settings, and was little used until around the mid to late 1800's because there were no tools strong enough to work on it!
Small sizes were common in the Victorian era, victorian ladies women generally had tinier fingers and waists!
I think it might be a good idea to have the ring appraised, especially as it has been handed down through the family.
