law: Indian Post Office Act, 1898 
  15. Official mark to be evidence of amount of postage 
  The official mark on a
  
  postal article denoting that any postage or other sum is due in respect thereof
  
  to the Post Office of 5[India] or to the Post Office of the United
  
  Kingdom or of any British possession or foreign country, shall be prima facie
  
  evidence that the sum denoted as aforesaid is so due.