By reading this forum I now know that this Couvert comes from Middelburg. The cover has only two markings, 1st the city sign with year letter, 2nd the master mark that I do not know, because I do not have the books. Would like to get help with this.
Hi Euser, Town mark Middelburg, late 18th century double headed eagle uncrowned, 1st standard or Grote Keur 10 penningen and 12 greinen or 775/1000 fineness. Year letter Y combined with the Town mark for the year 1798. Maker's mark IWG in a trefoil for Johan Werner Gericke, born in Hildesheim 25th March 1746, registered silver smith in Middelburg 19-11-1782. Died in Middelburg 27th November 1801.
Peter.
Source; J. de Bree Zeeuws zilver Interbook international B.V. Schiedam 1978