Read the stamp
958 hallmark
With a 958 hallmark, 958 means 958 parts silver per thousand. The traditional British Britannia figure denotes this higher standard.
Reading the number in context
958 means 958 parts silver per thousand. The traditional British Britannia figure denotes this higher standard. Britannia metal, a tin-based alloy, is a different material and can be stamped EPBM when silver plated.
What to check next
Compare with sterling at 925, or 92.5% silver. A stamp inside a company trademark can be a model number, so retain the neighbouring metal and assay marks. Use the maker signature and country device to continue the identification.