The old oval & octagonal style were used from about 1935 through 1968; however the keyway slot was revised for 1967 and more combinations added. The heads changed for model year 1969, the year GM started with thr tranmission-ignition-steering interlock (wasn't federally mandated until Jan. 1/70).

1967-on can all use the 1969-on keys, but to be correct the 1967/68 used the old style heads with the new style slots (making those a 2-year run only).