On 12 January 1915, British troops occupied Mafia Island for use as a base for the air and sea assault on the German cruiser S.M.S. Königsberg, which had retreated to safety in the delta of the nearby Rufiji River. Upon occupying the island, the British used a handstamp to overprint German East Africa issues for use by British forces in the area.
Due to the use of 1901 and 1905/1920 Yacht Issues, there are a mix of watermarked and unwatermarked stamps. The 1 Rupie and 2 Rupien values are unwatermarked, and all others are watermarked.
The first issues were overprinted 'G. R. / MAFIA', and they can be found with black ("a" suffix) or blackish-violet ("b" suffix) ink.