The country was called British Guiana until independence in 1966 when it became the Co-operative Republic of Guyana. That spelling might help when you are Googling its history.
The BG Militia was transformed into the British Guiana Regiment of the South Caribbean Force when the United Kingdom War Office assumed direct control over colonial defence during World War II. This regiment comprised seven companies of the British Guiana Home Guard; four were disbanded in 1945 after the war ended and three were retained under the name British Guiana Volunteer Corps. From here. And also orbat British Guiana on the 3rd September 1939 here.
