mb's theory of an "ancient spelling" is probably correct. Old English (Anglo-Saxon) had the letter yogh, which represented the sound of a modern Y at the beginning of a word and looked rather like a modern Z (actually, it looks more like the Cyrillic equivalent of Z). Later Scottish scribes, unfamiliar with the older alphabet, sometimes used z to represent the sound of yogh, e.g. in words like capercailzie (a kind of grouse) or the Scottish legal term tailzie. In England, similarly, the Old English thorn (ț), which stood for "th", was replaced by Y (hence "Ye Olde Tea Shoppe"). So I'd guess that Zuill is a variant spelling of Yuill, with the yogh replaced by a Z.
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