Metal Men comes to an end with issue 56 (Feb/March 1978), another Conway and Staton story, with an Aparo cover.
Gold, Platinum, Iron, Lead, Tin and Mercury find themselves under attack by the Inheritor, the newest creation of Doc Magnus. The Inheritor is a very powerful robot, the ultimate android, who has taken the concept that he is meant to replace the Metal Men literally, and is now trying to kill them. The team manages to get away from the Inheritor, but wind up out of the frying pan and into the fire when the military come back, insisting the team belongs to them. For his part, Magnus tries to convince the Inheritor not to kill the Metal Men, but the android will not listen.
The Metal Men then turn to Wonder Woman for help, in her civilian guise as Diana Prince, who works at the United Nations. Through her, they arrange to have a sort of trial in front of the Security Council, to determine if the Metal Men should be accorded the rights of being considered human.
Of course, that is when the Inheritor chooses to attack them, and the team defeat him, saving the diplomats as well.
This makes it a shoo-in for the UN to decree that the Metal Men are human, and make them citizens of the world, free from the control of the US government. Doc Magnus shows up as well to cheer them on, and things clearly get all patched up between them, as the robots are back with Magnus as of their next appearance, in Showcase 100.
Though the Metal Men have had a couple of miniseries, and make frequent guest shots, this was the last time, to date, that they had an ongoing book.