Here is the run down.
Except for a few very rare errors:
ALL business strike quality S mint Ikes are 40% silver
ALL proof 1971 and 1972 Ikes are 40% silver
SOME proof 1973, 74, and 76 Type I Ikes are 40% silver.
SOME proof 73,74, and 76 Type I Ikes are coppernickel clad
ALL proof 1976 Type II, 1977 and 78 Ikes are copper nickel clad
ALL P and D business strikes are copper nickel clad.
The rare exceptions:
There are two coppernickel clad 1973-S business strike quality pieces known.
There are about a half dozen 1974-D and 1977-D ikes known in 40% silver.