The jacket is from Annette Görtz. It closes with several large sew-on snaps (I always think of Tany now whenever I see sew-on snaps, although in this case the snaps are not covered).
From the functional side, the snap looks normal. It's sewn over a strip of constrasting petersham ribbon. (BTW: In the current issue of Threads [#136, Apr/May 2008], Kenneth King describes the strongest way to apply a sew-on snap - with a buttonhole stitch.)
The reverse side of the fabric is where it gets interesting. It looks like an incomplete circle in contrasting thread was stitched by machine where the snap is placed. Also, the contrasting thread that was used to apply the sew-on snap shows up as little 'bars'.
I can only imagine how carefully one must work to achieve such perfect-looking stitches!