Yep it's a flat button. they made some of those things so small I don't know how you could use them. Kind of hard to pin down an exact date but probably 1790's to 1820 or so
If you want to try and remove the crud and not remove the patina, simmer the coin on the stove in some diet coke. Take it out every hour or so and see what you can get off with a toothpick.