Re: Button Experts needed... never seen one of these!
It is a cuff-sized US Marines button. The gold gilt means it was for the "Dress" uniform, not the Combat/Fatigue-duty uniform (which has dull-finish buttons). The "Horstmann Philadelphia" backmark's lettering does not have serifs on the letters, indicating 20th-century manufacture.
Edit: To prevent anybody from misunderstanding my dating of your button's backmark. There are other versions of Horstmann backmarks with no-serifs lettering that date back to as early as 1865. But your button's exact backmark ("Horstmann Philadelphia") with no serifs dates it to the first half of the 20th-century.