sixta [OE]
sixte [MI]
sixth [ME]
sixþ
https://www.etymonline.com/word/sixth#etymonline_v_24137
https://www.merriam-webster.com/dictionary/sixth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED40596
https://bosworthtoller.com/27904