Ebenezar McCoy is descended via the Master/Apprentice lineage from the original Merlin. What if it was also by blood? The way Eb treated Margaret hiding she was his daughter may be a family tradition. If so the McCoy name or variant could have been applied by Merlin to hide his son or grandsons identity, an ancestor of Eb. McCoy means son of Aodh, a Celtic fire deity, and there are a dozen or so variations in modern usage. This is before Bob became a Wizards Research Assistant, but he would have been around before then in Faerie picking up information until one day he picked up the wrong bit of info, so he could have seen the son/grandson etc staff, or several different generations staffs and had enough examples to compile a thematic style.