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William Dugdale. Monasticon Anglicanum, new diutiuin, Vol. II, London, 1819, p. ...
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William Dugdale. Monasticon Anglicanum, new diutiuin, Vol. II, London, 1819, p. 54, footnote y, Google Books: "A.D. m.c.xxx. G. de Clare comes Gloucestriae et Hertfordiae obiit in nocte sanctorum Crispini et Crispiniani apud Penros in Brittania"