The 'stern piece' in dutch called 'Spiegel' translate from Mirror, an elaborate woodcarving pirated from the English will be permanently returned to England.
Most likely from an Admirals ship of the line, it maybe as the Admiral looked out of the stern he sees the reflection in water, hence 'spiegel gezicht' or Mirror image, my best take on the story. Prince Alexander of the Neth. also aboard will officially return the pirates price as a friendship gesture.