According to the button backmark dating book ("American Military Button Makers And Dealers; Their Backmarks & Dates"), the City Button Works (NY City) started making small quantities of "Great Seal" US Army buttons in 1907. During World War One the company devoted its entire plant to making uniform buttons. It did that again, making only military buttons, insignia, and military-medal ribbon bars during World War Two.
"Great Seal" US Army buttons made during the World War One era to 1923 had what was called "black finish" (which is not a paint) applied to the button's brass front, to make it less visible to the enemy. Your button dates from that time-period IF it has remnants of "black finish" on its front. If it doesn't, your button with the "City Button Works" backmark was manufactured sometime from 1923 through World War Two. (That company went out of business shortly after the end of WW2.)
