Difficult to assess a value for this.Forgeries of the stamp with the 1948 overprint are known.They could be scarce or very common ???
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It may be the 1948 Nov 15 printing on the envelope and stamp correspond to the (Ceylon Citizenship Act No. 18 of 1948) which became law on Nov. 15 1948. It denied citizenship to a vast majority of Tamils.This could be the reason the First Day Cover part repeats the date as the start of the rebellion..
The stamps are from the Tigers Eelam rebels issue (1986).The Tamoul region
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This was in a way an autonomous region(never officially recognized) from around 1987-2007.It was one of the most weoponizd conflicts in the world.
To this day stamps in support of the rebel cause have been denounced by the Sri lankese govt.
http://www.eelamview.com/2012/01/08...lent-speech/http://archives.sundayobserver.lk/2...08/fea08.asp