The circumstances and documentary evidence for the confirmation of Magna Carta during Simon de Montfort’s great parliament in 1265 have been fully examined  by Sophie Ambler.1 It was on 14 February 1265 that the king’s adherence to Magna Carta was proclaimed in the chapter house of Westminster Abbey.  At the end of the parliament, the full text of the 1225 Charter was sent to all the counties in an inspeximus, witnessed by Montfort and his leading supporters, and dated to 13 or 14 March. This was the first official circulation of the text since 1225.  Henry's failure to circulate it earlier, most notably at the time of the great confirmation of 1253,  was one reason for the unpopularity of his rule in the shires.  

No engrossments of these March letters are known to survive, but three copies with the full text of the Charter have so far come to light.2 These are in a statute book, probably written before 1297 (British Library, Harley 489, fos.4-10v), a Reading abbey cartulary (British Library, Harley 1708, fos.7-9v)3 and the Register of Richard Swinfield, bishop of Hereford from 1282 to 1317 (Herefordshire Record Office AL 19/2, fos.105-106).4  In addition, in two statute books there are copies, probably in the same hand, of the 1225 text where, after  the first three witnesses, there comes the statement that the Charter has  'given' on the 15 February in the forty-ninth year of the king's reign, so 15 February 1265. Since this is the day after the proclamation of the king's adherence to the Charter in the Westminster chapter house, these may derive, as Sophie Ambler has suggested, from a draft made with the intention of immediate issue, although, in fact, as we have seen, nothing was issued  until  the end of the parliament in March.5

If these copies of the Charter can be trusted, and I think they can,  they reveal that two significant changes were made to the official text.6 The first of these was the reduction of the baronial relief from £100 to 100 marks. The latter sum appears in the text Edward I issued of the Charter in 1297, and it has been thought the change was introduced then.7 But in fact, as these copies show, the 100 marks were there already in 1265. The second change seems never to have been noticed at all, namely the earl owing his relief not 'de baronia comitis integra', as in 1225,  but 'de comitatu integro'.  This is then found in 1297 and in all subsequent iterations of the Charter.

As we have seen, the 100 mark relief for a barony and the earl's relief being 'de comitatu integro' are found in numerous unofficial copies of the Charter. Since most of these are post 1265,  they may have derived from the changes introduced in that year. On the other hand, the idea that the baronial relief should be a 100 marks was an old one. It appears in Wendover's version of the 1225 Charter and in two other early copies,  Rawlinson 641 (above pp.20-2)  and British Library Add. MS. 25005 (above pp.68-9).  It is possible, therefore, it was included in drafts made at Runnymede. The way the text of the Charter dealt separately with the relief of the earl and baron suggests as much. The variant 'de comitatu integro' is also found in at least one copy of the 1215 Charter made before 1265 and certainly containing draft material going back to Runnymede.8 So 'de comitatu integro' may well have been in the air then as well.

The benefit to the baron in the reduction of his relief from £100 to 100 marks (£66 134s. 4d.) is obvious. There was also, arguably, a benefit to the earl in the changes made on his account.  Henry II, as Nicholas Vincent has shown, had never accepted that the title of earl was in itself hereditary and John had acted very much in that spirit.9  The 1215 Charter seems to make a concession to the king’s view, and has the earl succeeding not to an earldom but to the ‘barony of an earl’.  In the 1265 text, by contrast, the earl succeeds to his whole earldom, which surely means both to the title and also to the attendant estates.  A relief owed ‘de comitatu integro’ was both for the title of earl and  ‘de baronia comitis integra’. It was at least debatable  whether that was true the other way round.

It may be that the introduction in 1265 was simply due to the initiative of a clerk, either confused as to the genuine text or  knowingly making improvements.  On the other hand,  one cannot rule out the possibility that the  changes were made quite deliberately by Montfort and his ministers.  Montfort himself and his only certain comital ally, Robert de Vere earl of Oxford, would have welcomed the new wording over earldoms, just as Montfort’s baronial supporters would have welcomed the reduction in the baronial relief.