375, anchor, set of scales, Hallmark identification

I have inherited a ring from my Aunt which looks like a cluster of 8 diamonds with an amethyst centre stone, from what I can make out the hallmark on the gold band is as follows:

The letters OPT followed by a crown followed by the digits 375 followed by an anchor on it's side followed by a set of scales (like the star sign libra, but with the digits 375 across the upright) followed by the letter a .

Please could you offer some information on what these mean.

Yours sincerely
Miss P.Tudor