The aircraft's right main landing gear retracted during the landing roll. During initial examination of the gear by the FAA, it was noted that one of the two clevis fittings had separated from the gear casting. It remained attached to the rod end bearing on the push/pull rod of the extend/retract cylinder. Two persons pushing on the landing gear drag brace attempted to unlock the system. They were unsuccessful. The downlock nitrogen charge was bled off and then one person could push the drag brace enough to activate the gear light and horn in the cockpit. After the other downlock devices in the form of bunger cords were removed, light pressure on the drag link activated the unsafe light and horn. This action took place after 1/8 inch movement of the drag brace. One inch movement was necessary to 'unlock' the gear for retraction. The drag brace was examined for evidence of some object having hit it with enough force to unlock it during landing. No such evidence was found.