It's a civilian flat button 1810 -1830 or so. I think I see a ring on the back so it probably had a back mark but I don't know that you will be able to make it out. Buckleboy has a good post on cleaning buttons. you might want to check it out. We dig flat buttons in civil war camps, both US & CS all the time. We find them in later areas too. I dug an 1820's flat button in a logging camp in the same hole with a 1927 penny. I have no doubt that even up into the 1940's, or maybe even later, a person would not have thought twice about pulling an old flat button out of the button tin or box and sewing on some old coveralls.