I would like to add one more point ...
Sec. 260 - Additional Director shall hold office only UPTO the date of next AGM. (As the word 'UPTO' is used, Additional Director can be regularized anytime before forthcoming AGM)
Sec. 257 - if Additional Director is to be regaularized, special notice before 14 days of GM is required.
Since Additional Director is not a retiring director u/s. 255-256 and hence complinace with Sec. 257 is necessary.