So the story goes - Steve Rogers, a sickly scrawny patriotic young man who so wanted to fight for his country after watching the war newsreels decided to volunteer himself for Operation: Rebirth - to become super human by an experimental serum in order to aid the United States war effort. Lo and behold, the Super-Soldier was created but the scientist Abraham Erskine was killed before he could ever pen down his secret formula. Thus there was only one Super-Soldier and Steve Rogers became Captain America and the rest as they say, is history.
What possessed Toy Biz to produce figures with their mouths hanging open is beyond me. And the star on the uniform doesn't have to be THAT big, even if Captain America is best known as a patriotic super hero - that's really overkill.
Perhaps the open mouth expression is for Cap's famous rallying cry "Avengers Assemble!!"
Be sure to check out Captain America by Hasbro