Hello.
The AB stands for Atkin Brothers. The S for Sheffield. The flower-like mark is just a pseudo mark.
http://www.925-1000.com/silverglossary4.htmlThe Atkin Brothers began using your mark in 1853.
In 1890 they introduced a fleur-de-lys mark, often in addition to your mark. Check here:
http://www.925-1000.com/silverplate_A.htmlSo, I think your set dates to between 1853 and 1890.
The number 6477 is just a catalog or pattern number.
The letters WM above could be those of the retailer.
However, they might have been the initials of the owner.
Were your great Grandmother's initials WM ?
Pat.