In the Post-16 Year Interlude period of ROCH, Yeung Gor did not always carry the Heavy Iron Sword on his person. He had mastered the Sad Palms and that was adequate for dealing with most enemies. Hence, he did not have the sword available to him during that final confrontation against the Golden Wheel Monk at Seung Yeung.
On such occassions, where did Yeung Gor park the Heavy Iron Sword when he wasn't carrying it? He was a wanderer. There was no closet or vault he could store it in.