The Combined Nomenclature (CN), v1:2018
9704 00: Postage or revenue stamps, stamp-postmarks, first-day covers, postal stationery, stamped paper and the like, used, or if unused, not of current
https://www.dst.dk/en/Statistik/dokumentation/nomenklaturer/kn?id=bdccfab4-a71c-4f32-8b96-d3d9ed56013c