The area of present-day Cameroon was claimed by German as a protectorate during the "Scramble for Africa" at the end of the 19th century. During World War I, it was occupied by British and French troops, and later mandated to each country by the League of Nations in 1922. The British territory was administered as two areas, Northern Cameroons and Southern Cameroons.
Cameroon became independent in January 1960, and Nigeria was slated for independence later that same year, which raised question of what to do with the British territory. After some discussion (which had been going on since 1959), a plebiscite was agreed to, and held in February 1961. The Muslim-majority Northern area opted for union with Nigeria, and the Southern favored Cameroon.
Northern Cameroons became a region of Nigeria May 31, 1961, while Southern Cameroons became part of Cameroon on October 1.
In the meantime, the area was administered as a United Kingdom Trust Territory.
The territory had long used postage stamps of Nigeria, but in order to avoid the appearance of bias, the government issued the current Nigerian stamps overprinted with CAMEROONS / U.K.T.T. in red. These went on sale October 1, 1960. Although available in both northern and southern areas, most usage was in the southern area, northerners continuing to use Nigerian stamps. These stamps became invalid in each area when its separate existence ended.
The stamps are readily available to collectors today, the lowest values all selling at minimum price used or unused, with the 1-pound stamp of the series selling for about five US$.
(Note that the illustrated shilling value is postmarked August 31, 1961. This is not technically a valid usage, since since the Northern Cameroons town of Mubi had become part of Nigeria two months earlier, but the stamp underneath the overprint was still a normal Nigerian stamp, and so either the clerks at the post office didn't notice the overprint, or else they simply let the stamp pass through.)
Postage stamps
Threepence stamp used at Kumba,
now in Cameroon
Shilling stamp used at Mubi,
now in Nigeria