The worst (or best, depending on one's viewpoint) I saw was a navy holster/stock that was bolted to a wall, by drilling some large holes through the stock.
It was wrongly identified as being an 'artillery holster/stock' as well
This mutilation was on display at a small museum on the island of Terschelling.
A possible explanation for the 'pimped up' gun you saw: There was a special arrangement for members of the former resistance, who were allowed to keep their guns, provided they were registered (like all legal guns here). Some decided to 'improve' their guns by adding things like the ivory grips you saw, or by refinishing / restoring the guns, etc... Recently I saw a rare example of a German Kriegsmarine M1934 pistol. The Eagle/M was removed from the slide, and the owner had engraved his initials at the same spot. Other guns survived relatively well. I have several examples of Dutch resistance pistols that survived in their original configuration. But most of these ended up in private collections in the 1960s-70s. The gun in question was probably loaned or donated to the museum by relatives of a former resistance member who didn't want to retain the gun after his passing.
As I said, the country has had no tradition of gun ownership since 1919, when the current gun laws were first introduced as a method to keep firearms away from members of the communist parties (the government feared that something similar like the communist revolts in Russia and Germany could start in the Netherlands as well). As usual, the law fulfilled its purpose and was then left in place for eternity