The 'cheater' chunk sight, is a tall U notch close to the breech, with a bead front sight. The U notch is used as an aperture sight. The rules state, iirc from Don, Getz teachings, that the top opening of the notch cannot be tighter than it's greatest width.
The bead was held in the bottom of the U, equi-distant from the sides and bottom. With minor amount of practice this sight is no different in actual use, from any aperture sight. there is simply not a top to it - practice is all that is needed.
The sight's description as such, was shown to me in E-mails a long time ago through pictures of the actual sight. I no longer have those pictures however the rear sight was adjustable for both windage and elevation.