This was the designation of Allied military ferry convoys (together with a numerical suffix) plying the route from the Clyde river to Reykjavik, and as such reciprocals of the 'SD' series (April 1941/May 1945).
Each convoy comprised a small number of transports carrying troops and military supplies, the latter including aircraft destined ultimately for delivery to the USSR. The series ran at approximately monthly intervals, the troopships and merchant vessels often returning in the reciprocal 'SD' convoy of the same number.
The first of these 71 convoys was DS.1 of 10/11 April 1941 with the 3,244-ton British infantry landing ships Royal Scotsman and Royal Ulsterman escorted by the British destroyer Lancaster and escort destroyer St Mary’s. The last was DS.65 of 8/12 May 1945 with the 14,182-ton British troop transport Largs Bay escorted by the British destroyer Westcott and frigate Rother.