FWIW... The four-letter marks indicating base metal (viz., WMFN for nickel silver, WMFM for brass, and WMFB for Britannia metal) were used from 1880 up to WWI, which is consistent with the set not being found in the 1925/26 or 1937 WMF catalogs. However, it's not found in the 1906 catalog, either. M...