23mm, Civil War era Army Corps of Engineers coat button "Scovills & Co/Waterbury" rmdc backmark.  This is quite rare with a raised-mark-depressed-channel backmark.  It has been in my own collection for years and is out of the fabled collection of David F.  Johnson author of the 1940's reference works (Uniform Buttons of the American Armed Forces 1784-1948) on buttons which preceded Albert's book. It was listed as 117-B.