Imperforate Belgian Stamps With Numbers On Reverse

Quote:
The below quote was taken from an article by Henry Gitner Philatelists titled "Belgium Imperfs and Belgium Proofs."

For every modern stamp it issues, Belgium provides the philatelic world with some of the most seldom seen specialized material. For most 20th Century issues and almost all from 1930 to date, IMPERFORATES have been prepared in the original colors and distributed only to high government officials. From 1929-1970 only 200-400 of each were printed. From 1971 to date, 1,000 of each have been printed. However, only 240 of each of the Parcel Post Imperforates were ever printed!