I think the piece would need the other necessary assay marks in addition to the 'Mappin & Webb, London'. Maybe this is to show the different mfgr's marks used by M&W, but it's misleading, IMO, since the stamp could be used on plate as well as sterling.

Because Mappen and Webb only made sterling....and those were the various "makers marks" they used...
In hallmarked British silver...all marks must be present...the maker's mark, the assay mark insuring quality, the city mark denoting the place where it was made and the monarch's head, denoting who was in power when the piece was made and the date letter.
Plated ware didn't show those marks.
Linda C

Mappin & Webb also made plated wares, however the sterling pieces would have to have English hallmarks along with the maker's mark which is the one you are questioning.
Their plated wares had only maker's marks on them.

Here is a link that might be useful: Sterling & plate marks for Mappin & Webb