A very fine used example of the 3d purple on deep yellow King George V stamps from the 1914-1922 De La Rue Imperium keyplate design, multiple crown CA watermark on chalky paper, with a yellow backing. The SON cancel was in Lagos, and is dated Feb 3rd 1920.
Included in the photos is a scan of every different paper back found for this paper color.
Gibbons lists a very fine used example at 7.50 pounds, (approximately $15). The stamp offered here grades 80 as follows:
