What do the stamps WM a crown 18 an anchor and b refer to on a gold ring?

found this--- seems to indicate British origin. The final inital is a clue to the year, but I don't know what those are.

http://en.allexperts.com/q/Jewelry-Making-3236/grandmothers-ring.htm

hello there - i am in canberra and my fiance recently found a ring in the veggie patch with a similar insignia. the jeweller said it was over 150 years old, was brittish with the royal crown, and 22 carot gold... that's all we know. want to make contact?