On a Type II hammer gun, there is no joint chaeck as was found on the earlier Type I hammer guns. The only thing that stops the barrels from opening past normal is the fore end iron hitting the frame. If care is not used in opening, a great deal of force is applied to the fore end lug, and it is torn from its seat on the barrels. It is only held in place by a small diameter screw and soft solder where it is attached to the barrels. Usually when the fore end lug is moved, the rib in front of it is buckled, and that sounds like what has happened here.