Here is a complete strip of 4 airmail labels from British Guiana. The color of labels is more greenish then blue as you see in the picture. It's very difficult to get correct color with my camera.
Year: 1933, Ungummed. Günter
Mair's catalogue no: - BGA-A-3b, perforated H 11, vertical strips of 4, lower label of several strips comes with ref.: "P.O.-311.", black/light greenish blue, 5*****