In guns for sale "SIGWolf" has a 6906 for sale and has pictures of the gun and mags with the "plus 2 adaptor" on them, was about half way down the first page.
The original 4013 had 9-round mags. The newer 4013 and 4013TSW were 11-round mags. They are not interchangable. The floorplates for the 9-rounders come in two configurations: flat bottom and finger extension. Neither adds any ammo capacity. CDNN sales both types.
Actually the original non-TSW 4013 uses a single stack 8rd mag. The TSW series uses a 9rd double stack finger rest mag. The full size 11rd mags from the 4006 fit and the "Plus2" adaptor just fills out the grip area where the mags hangs down on the compact frame. The adaptor also works with 5900 mags in the the 6900 frame guns, but then it's a "Plus2 and 1 more adaptor".
I resurrected this thread for the fact that I finally found one of these extensions. It uses a 4006 magazine and makes it compatable with the 4013TSW. The picture has a standard 9 round 4013TSW mag on the left, the 11 round 4006 with spacer in the middle, and a 11 round standard 4006 on the right. Don't really know where to find another one.