Netherlands East Indies. (Singapore). 1940. Air Mail envelope addressed to Amsterdam bearing Netherlands Indies SG 405, 20c purple tied by Bandjermasin date stamp '24th April 40', route via Singapore with 'Passed by Censor' label tied by framed 'Passed By/ 51/ Censor' cachet with circular '21' censor cachet in violet on face. Scarce single censor numeral from Singapore.
The single ring numeral censor appears to have been applied in transit at Singapore in early 1940.