Prevent black money
If you don’t declare your Guldens with the tax authorities, it becomes black money. If your Guldens would increase in value significantly in the coming years you will not be able to declare/explain it and you would end up holding a large amount of black money.
To avoid problems, it is therefore desirable that you declare your Gulden assets to the tax authorities.
Because each country has its own rules it is not possible to give you more information. For the Dutch, an explanation in Dutch is available for other nationalities, it is recommended to obtain information from your national tax authority.
This post is also available in: Dutch