Great looking button. How did you come across this? This could possibly be a state deal button, but I'm just guessing. Maybe someone will come along with a button book and help you with an ID. Congrat's to you.
It is most likely a state seal button but probably from the late 1800s to early 1900s. I could not find that exact button in either Albert's or Tice's books. Plus, that particular backmark was used from 1880-1920.
The two bigger pictures of the button aren't of the actual one I found. Mine is the dirty, darker one in the 3rd picture above. I just used the other two pics for reference as it's the exact same as the one I found, just much better condition lol. Couldn't find this in any of my civil war relic books either.