Thank you for making and posting the additional photos of your button. They show that its original dome height has been reduced by crushing, and its brass back is missing. That makes it very nearly impossible to time-date.
I should also mention:
In your new photos, it appears to a version of brass 2-piece button called a "false 3-piece" (or "false staff") -- meaning, a 2-piece button manufactured to visually resemble a 3-piece, but it's actually not a 3-piece, which has a separately-made rim crimped around the edge of its front. Although the book "Record Of American Uniform And Historical Buttons" by Alpheaus H. Albert shows two varieties of false-3-piece US Army Staff Officer buttons, there is no EXACT match-up for yours in that book.