It looks like this one never got cleared up, so here goes:
The 1966-earlier keys used the same keyway for both head shapes (this pattern goes back to the mid 1930s). In other words, you could cut an ignition key (normally octagon head) on a trunk key blank (round head) and it would still work just fine.
1967 was the first year for the "new" GM keying system. The two keys now had different keyways, with the ignition/door stamped "A" and trunk/glove stamped "B". In 68 the keyways changed again, and were stamped "C" and "D". They changed again in 69 to "E" and "H" (these were the first of the large heads because of the change to steering column locks) and again in 70 to "J" and "K". For 71 they started over again with the 67 "A" and "B" keyways, except that these keys had the large heads to work with the column locks (they will still work fine in the dash-mounted switches). This sequence repeated every 4 years until GM changed to the double sided keys.
Note that blanks with keyways "A", "B", "C", and "D" were made with both small and large head styles (and are still available that way if you are doing a resto for points). "E", "H", "J", and "K" were supplied only with large heads (at least by GM - I suppose anything is possible from the aftermarket suppliers).
If you need more info, feel free to ask.