The weapon is found on a bed in the back of the supply plant in The Pittsteelyard close to a single steel ingot. Enter the plant, then go east through the hallway. Turn north and continue until you reach two exits. Take the left, continue through the passage, climb the stairs and eventually exit into a large room. There's a bed on the left as you enter. The Man Opener is on the bed.
Behind the scenes
The name is a pun on the name of a common kitchen tool, a can opener.