In short, this is a novelty marker/flag holder that someone either purchased and erected to recognize an individual, or it was erected by a local veterans association. While it appears old, it is not original to the time period and was erected long after the original burial.
FCL stands for “Fraternity, Charity & Loyalty”, the three guiding principles of the Grand Army of the Republic (GAR), which is is a fraternal organization of veterans of the Union Army who served in the American Civil War.
The two symbols are the anchor (for sailors) and crossed rifles (for land forces). This design was said to be patterned loosely on the original GAR badge.