"Used abroad" - Indian stamp used in Singapore

Indian stamps were used in many places outside India (the international term for this is “used abroad”), including Aden, Nepal, Burma, various Persian Gulf states, etc. This also applies to Singapore, the British port of departure for mail from Southeast Asia to Europe. Single stamps can usually be determined by the cancellation if they were used in Singapore. However, not in cases where a stamp is cancelled with a mute cancel (standard type, diamond-shaped with 9×9 dots), which was used for cancellation in numerous Indian post offices. 

This is only possible if the stamp is affixed to a cover that clearly proves the origin “Singapore”, at best by a local postmark next to the stamp or on the back of the cover, as shown. Apart from the fact that this mute cancellation is little known when used in Singapore, the fact that it is struck on a 4-cornered 4 Anna stamp from 1854 makes the cover an exceptional rarity.