That is so cool, I often wonder if my grandfather would have such old coins in his possession as well. I love reading this type of stories and recently, I've been browsing for old coins (not just the Dutch) and hope to get my hand on some in the future.
A few weeks ago I was checking the boxes with coins on the flea market and found a knaak as well. I bought it for sentiment, not perse for a certain value as I Had no clue what it would be. The guy asked 1 euro per coin, so I let the others for what they were as they weren't Dutch so I had 0 knowledge, at least the Knaak held some sentimental value :)
!DHEDGE
Yeah it's fun! Did your grandfather also collect? I haven't checked my grandfathers collection for a very long time, but if I don't forget, I'll check it next time. IIRC he had some really old stuff there. Thanks for the comment, happy holidays!