Your button seems to be a Great Seal Button that started use in 1902, and is still in use today. Most of the ones that are dug, would be from the WW1 or WW11 era. There are some slight differences in the buttons, and the makers depending on when they were made. Military finds are always great finds in my book. Congrats.
According to Bigcypresshunter's research your backmark dates the button to WWI confirming nhbenz's ID. Here's a link to BCH's thread on Great Seal Backmarks. Go to page 25 for the latest list.