Interesting! Looks like a common failure. Inside the housing the sheared off there are two rubber blocks holding small steel bars that ride on each side of the long arm. As they pass over the wider parts of the arm, the rubber is supposed to compress increasing the friction of the small steel bars on the arm, which provides the resistance to the door closing. However, the rubber blocks get hard after 30 years so the friction on the wide parts of the arm becomes so strong that the leverage of the door force on the housing breaks it off the mounting plate.
Not only is this GEN I part not available from any Montero supplier, it is not even identified by a part number in the Mitsubishi shop manual or Gen ! parts list.