So here is my theory. Terra has a belt buckle that looks like the heartless symbol and he ultimately gets mixed with MX in the end and make "Terranort". Since "Terranort" was the one doing research about the heart and made heartless (also a heartless of himself). So he probably made the symbol too. If he* made the symbol he probably chose terra's belt buckle from his memory and made the symbol. So the belt buckle was probably not a coincidence on Nomura's part.