Indeed year letter script l for 1870.The office mark or Minerva, if I'm correct shows the regional assay office letter A in her helmet for assay office Amsterdam. The maker's mark appears to be an over-struck.
I do recognize an anchor in the maker's mark and it could be for the widow of Hendrik Helweg. Maiden name Maria Jacoba Frederiks perhaps over-striking another maker's mark. For more information see;viewtopic.php?f=5&t=28590#p81398