Okay, yes, the thinner diamond seems to be endemic to the trade dress of Tarzan for its entire run -- Nick Pope's scans show it on 11-13 and 15-16, changing to the fat burst (taking the place of the "Still 35¢!" copy from the newsstand Marvels) by October 1978. And there are other titles as well, as you note.   Looking at what series did and didn't have narrow diamonds pre-1979, I see a possible common thread. It appears to be only the titles that started while the Whitman experiment was alre