Not really an unknown soldier, for his name and rank in the army of the Prince of Orange is named on his moarning sign. There is not a lot more known about Frederik Willem van Meyers, died in 1742. The restoration of his moarning sign has been a precise job, done by Randolph Algera of Art-Decor.
See more: https://www.omropfryslan.nl/nijs/889560-rouboerd-fan-unbekende-soldaat-yn-martinustsjerke-gytsjerk