South Africa. (Netherlands East Indies). 1939. Air Mail envelope addressed to Medan, Netherlands East Indies bearing South Africa SG 57 tied by Durban roller '8th April', underpaid with instructional 'Insufficiently Prepaid For/ Transmission By Air Mail' in violet on face and circular 'T/25c' hand-stamp and bearing Netherlands East Indies postage due SG D227, 2 1/2c salmon and SG D231, 10c salmon tied by Medan date stamp '16th May'. Lovely item of underpaid mail from South Africa.