Hofvijer Grey fountain pen ink made by P.W. Akkerman pen store in The Hague.
Originally made to celebrate their 100th anniversary, this ink comes in a vintage-style bottle with a narrow neck with a marble stopper, to create a well at the top to aid in filling pens.
Hofvijer is a pond in the centre of The Hague. In the middle there is a small island with plants and trees which has no name, it is usually referred to as "the island in the Vijverberg". (source: Wikipedia) Ackerman