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; http://www.925-1000.com/forum/viewtopic ... 590#p81398
How is it possible that you've recognize the A in the helmet. Okay if you know you will recognize, but for me it was hard time to see. However it is there indeed and with the other information I'm glad as well.