Sorry Simon, can't see that the sail would be double thickness there. Seams are typically only 1cm of overlapped cloth. That 4th batten area is clearly of the order of 5-6cm. The batten pocket would account for around 4cm. If there is a seam above or below the pocket that might explain the appearance (a coincident seam) but it is still a shame as it does spoil the look of the sail.

Surely those are just black painted or anodised beams Wouter? I can't see the justification for the cost of suitable carbon beams. It's not as if it hasn't already been demonstrated that an F16 can be built down to weight without compromising structure.