New Pocket Watch 16S Mainspring for Elgin Pocket Watches. This is a new #817 size 16S mainspring for Elgin pocket watches. This mainspring is superior quality and leads the field. The edges are rounded to reduce friction in the barrel. It is oiled before going into the package and the wrapper is chemically tested to secure freedom from moisture. Dennisons of 17 1/2 x 4 1/2 x 21 1/2.