Here's a pic I took a year ago:

https://drive.google.com/open?id=0B6oZKv_ATziRRDd6SUM1Vy1CNEk

- when CLC came to Madison WI with an Annapolis Wherry, among various other craft.

Looks to me like that rowing rig attaches to both thwarts. I recall one of the CLC folks making it fast with a bolt & maybe wingnut before I took it to the water nearby.

   I've just finished installing one in my single wherry and I also found the manual directions aren't totally clear. It does indicate that the RoWing is through bolted to both thwarts and there are sketches to show the proper position. In my kit there were two 3.5" machine screws, washers, and wing nuts. I took the "Build Your Own" class and the instructor mentioned we would have to drill through the beam of the RoWing and the thwarts of the boat. That was easy enough at the forward thwart but at the rear thwart the RoWing has an extra heavy bracket in the way. I might have gone and bought a longer screw but I would have had to place it pretty tight to the vertical frame making installation dificult. Fortunately my RoWing had two inserts on the bottom at just the right position so I was able to cut the head off of the machine screw and glue it into the insert with thread-locker. It is now a permanent stud sticking down and can be bolted from below the thwart.

Cameron3, this is exactly what I did also. Manual is really vague on this.