It is a "Great Seal Of The United States" US Army button. Because it has a raised rim, and it lacks the "black" finish" used on those buttons until 1923, it dates from sometime after that year. We can narrow down its time-period even further if you can tell us EXACTLY what its backmark says. The Waterbury Button Company changed its name in 1943 to simply "Waterbury Companies, Inc." -- and changed the backmarking to say "CO'S" (the abbreviation for Companies). So, if your made-by-the-Waterbury-Button-Company "Great Seal" US Army button's backmark says "CO." it was made between 1923 and 1943. If it says "CO'S" it dates sometime from 1943 into the 21st century.