law: Companies Act, 1956
  Sec 91    -     Calls
  
  on shares of some class to be made on uniform basis 
  Where after the commencement of this Act, any calls for further
  
  share capital are made on shares, such calls shall be made on a uniform basis
  
  on all shares falling under the same class.
  Explanation. :   For the purposes of this section,
  
  shares of the same nominal value on which different amounts have been paid-up
  
  shall be deemed to fall under the same class.