Found at CTH3, this is a little button that still has the loop on the back, though flattened. It measures 14mm. I cannot make out any writing on the back but I think it might have had some at one time.
yep it's a US general service eagle button. That type was used from the 1850's to 1875. If it has a tin back it is more than likely post 1865 if it has a brass back we would have to know the backmark to narrow the date down any more.