When the stylistic changes were introduced for the 77 model year, one of them was the shape of the shaker hood, which had been unchanged since 1970. The intial 77 models had it setting rather low in the opening in the hood, and there was a pronounced ridge up the center of it longitudinally. Speaking candidly, most people thought the design sucked. Part way through the year it was redesigned and raised, so it looked like the one on your car. The redesigned version was more in line with the traditional location of the shaker relative to the rest of the hood, and it was much more appealing to folks at the time.
Somebody here will probably know when that running change occured (approximately), but that's it in a nutshell.
