Civil War button found in NY

[CMJ76]

One of my best finds so far. Anyone have any idea how old? Not sure why it posted pic sideways.
Thanks

 

[WVExplorer]

Nice find.

I’ve seen other posts on here identifying it as a federal general service button. More than likely Civil War time frame but I believe they were made a little before and after CW.

Any writing on the backmark?