Bingo! The genuine coins were minted at Bombay (with a small dot as the mintmark) or Calcutta (with no mint mark) in .9170 silver (just shy of being sterling). More than 200-million were produced at each mint in 1918. The dramatic difference in weight between silver and tin implies that this was created for some reason other than passing off as genuine in commerce - perhaps as a jeton/counter, or for use in a children's game.