law: Indian Stamp Act, 1899 
  26. Stamp where value of subject-matter is indeterminate 
  Where the amount or value of the subject-matter of any
  
  instrument chargeable with ad valorem duty cannot be, or (in the case of an
  
  instrument executed before the commencement of this Act) could not have been,
  
  ascertained at the date of its execution or first execution, nothing shall be
  
  claimable under such instrument more than the highest amount of value for which
  
  if stated in an instrument of the same description, the stamp actually used
  
  would, at the date of such execution, have been sufficient:
   35  [PROVIDED that, in case of the lease of a mine in
  
  which royalty or a share of the produce is received as the rent or part of the
  
  rent, it shall be sufficient to have estimated such royalty or the value of
  
  such share, for the purpose of stamp duty,- 
  (a) when the lease has been granted by or on behalf of 36[the
  
  government], at such amount or value as the Collector may, having regard to all
  
  the circumstances of the case, have estimated as likely to be payable by way of
  
  royalty or share to 36[the government] under the lease, or
  (b) when the lease has been granted by any other person, at
  
  twenty thousand rupees a year, and the whole amount of such royalty or share,
  
  whatever it may be, shall be claimable under such lease:]
  PROVIDED ALSO  that where proceedings have been taken in
  
  respect of an instrument under section 31 or 41, the amount certified by the
  
  Collector shall be deemed to be the stamp actually used at the date of
  
  execution.