They're the same HAPP arcade buttons on any american arcade cabinet, but having them on the side makes all the difference. I've been trying all kinds of control inputs for playing digital pinball, from traditional gamepad shoulder buttons to keyboards to arcade sticks, and nothing really felt satisfying. Finally, I moddified one of my existing arcade stick controllers with goldleaf HAPP buttons on the side... Wow, what a difference! The illusion is so much stronger, but why should it really matter if the buttons are on the top or the side? It seems trivial, but it makes all the difference in my experience.