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This is a cloth shoulder insignia from the British South Africa Police (BSAP) of Rhodesia, now Zimbabwe.
The letters NS stand for National Service, identifying a national serviceman serving with the police rather than a regular career policeman.
The BSAP's own regimental history records that the first intake of 42 National Service Patrol Officers began training in July 1973, at Morris Depot in Salisbury (now Harare). 
This suggests your shoulder insignia is associated with the 1973–1980 period, during the Rhodesian Bush War.
