Most of those old MX8 combo's came with 2 triggers. A top for the single barrel and a bottom first for use with the o/u barrels.
Since the trigger is stamped o/u and is top first it has either been switched from a bottom first to a top first...or the gun is not a two trigger combo.
Since the gun is factory choked for the bottom barrel to be the first to fire, I'd say the second trigger that came with this combo is missing or not available.
milt