The heading is ‘Carta eiusdem magna de Ronnemed’, the ‘eiusdem’ referring to King John, whose charter granting the church freedom of elections immediately precedes the copy of Magna Carta. The great cartulary or register of Canterbury cathedral, now known as Register E, was a work of the 1290s. I have argued that its copy of Magna Carta was made from the engrossment of Magna Carta now in the British Library and known as Ci (British Library Cotton Charter xiii. 31a).1
D. A. Carpenter, Magna Carta (Penguin Classics, 2015), pp. 477-80; C. Breay, J. Harrison and D. M. Smith, Medieval cartularies of Great Britain and Ireland (London, 2010), no.168.