[YNOBIL]

From 1935 to 1966 the shafts of the keys were the same for all keys. On these keys the door and ignition key was octagon and the trunk and glove box key was pear shaped. These keys had no letter designation on them. Beginning in 1967 the octagon ignition key had a new shaft shape and had an "A" designation. The trunk key an entirely different shaft design but was still pear shaped, it was given a "B" designation. In 1968 they kept the octagon and pear shaped, but created two new shaft designs. The octagon was a "C" key and the pear shape was a "D" designation. As we all know in 1969 GM moved the ignition switch to the steering column. This is when they switched to the square ignition and round trunk keys. They came up with a new shaft design for the square igntion and coded it "E". The round trunk became "F". From then on they stuck with the square and round keys. In 1970 they created a fourth shaft design with a new set of letters. In 1971 they stayed with the square and round styles but repeated the 'A" & "B" shaft design of 1967. In 1972 they repeated the "C" & "D" shaft design. I don't know what they do these days, but they rotated those four key series for years. Let me clarify I am refering to GM parts when I discuss key codes not aftermarket. I have the original GM part numbers if needed.