I don't have a loose '28-29 coupe/roadster rear fender to photograph, but I can show the shallow cutaway on a 1928-29 sedan fender (on the left), which is almost identical to the coupe/roadster cutaway area. They both meet the side plash apron at the same place, so I think the sedan photo is acceptable to judge the shallowness of the cutaway. For comparison's sake, I am also including a '30-31 rear fender cutaway photo (on the right). The difference in cutaway depth should be obvious to see why the 1928-29 and 1930-31 coupe/roadster rear fenders are not interchangeable. I imagine the radius that fits inside the rear fender well is different, too, but I can't confirm that.
Marshall