23mm, Virginia State Seal button with "* Extra * Quality." backmark.  This specific backmark is a notable exception to the general and often quoted rule that "large letters" in the backmarks are Post War.  This is a good, solid backmark.  It is found in Civil War sites on the reverse of MS35's and US Staff Officer buttons among others.  Civil War button collectors learn this mark and refer to it as "The Big Goofy Letter Extra Quality."