Not sure what that is either. Crumby image, and searching for photos online is gonna' be a pain with those words being so common. My guess is that's a shadow falling over the side rail (photo isn't exactly straight down).
Unless you find a restored or well kept boat (photos of), you won't have a good idea. Replacement seats are going to be relatively different, but you can find things that fit the design. Truly, the rigging - seats, windshields, helm, etc. - were typically not made by the boat manufacturer. Manuals don't really exist for boats, and the few that do aren't really that detailed (not like you'll find a parts list or anything).
Make it how you want it ...
