And I found this:
Quote:
Belgium has provided the philatelic world with some of the most seldom seen specialized material. For most 20th century issues and almost all from 1930, imperforates have been prepared in the original colours and distributed only to high government officials. Stamps up to 1950 were sold gummed or ungummed, as issued, sometimes with a script Specimen overprint. After 1950, all Belgium imperforates were issued ungummed with an individual number printed on the reverse of each stamp. Each of these imperforates is then unique. The Belgian post issued only 300 imperforates as gifts to VIP's. The Belgian King receives a block of 10, high-ranking ministers a block of 4 and the rest a single stamp. Philatelic magazines and catalogues highlight the rarity of these stamps.
https://applications.icao.int/posta...ns_issue.htmMaybe a good deal if Scott had only listed them. I wonder if other catalogs do?