It's just a superscript 6 (⁶) - MB can handle unicode in tags, so there's no reason it shouldn't be able to do a superscript or subscript. But the source I'm copying from may not be transferring the necessary info.
Edit: Ha! It worked when I used that specific unicode version. Problem partly solved. Although it would be nice if MB had a way to interpret superscript and subscript from other formats and convert them as necessary.