According to the very-detailed historical article at following website, the Metal Specialties Company of Attleboro Mass. began operations in 1916 and was incorporated two years later. See page 14 of the .pdf document at the link:
http://www.rmlonline.org/pdffiles/Stone%20chapter%20xiv.pdf
If your excavated Great Seal buttons show traces of "black finish" on the brass, they date from World War One to 1923. If there's absolutely absolutely no trace of "black finish" on your Metal Specialties Co. buttons, they date from 1923 to World War Two.