108 M is in a series of 3 buttons that are domed with a rim. The rim increases with the size. The first is the smallest 108 S, it is 5/8″ and a favorite with seamstresses for this period. price 1 to 12 @ .55 each, 13 to 144 @ .48 each and over a gross (144) .40 each.
Matching buttons are 109 F and 110 L. Dome slight with a rim. Generic.