First and foremost, 40% silver Ikes were minted in San Francisco. So look for the "S" mint mark. Silver Ikes are commonly called Blue and Brown Ikes. Blue Ikes were the business strike and sold to collectors in a blue envelope. Brown Ikes were the proofs and were in a brown display case. The problem comes in that San Fran also minted clad proofs starting in 73. They were only made in 71, 72, 73, 74 and 76.