This question comes up from time to time and as yet I have not heard that it has had a solid reasonable explanation. Assuming the Cobra Jet emblem on my 9/19/68 Metuchen scoop is original one can see the location and that it is glued in place. The 351 and 390 cars had a different, to the rear, location and were attached with two threaded bolts through two pre-drilled holes. The 428 emblem using the threaded bolt attachment method seems to appear and disappear in an unpredictable and random fashion. OE, assembly line, photo shoot or, my personal favorite, an owners desire to visibly proclaim the engine displacement of his 428 car who knows ? Brian