The Battle of Maldon survives only via an early modern transcript. The text of Maldon copied by Casely was part of a manuscript from Sir Robert Cotton's library. A catalogue of the holdings of the Cotton library was compiled in 1621, where volume 29 is noted as containing a copy of Asser's Life of King Alfred, various hagiographical writings (i.e. relating to saints), and a cryptic entry entitled 'fragmentum historia Saxonum in lingua Saxonica' (a fragmentary history of the Saxons in the Saxon language). It is this last piece which we now know as Maldon.
The manuscript was catalogued again in 1696 and 1705 providing more details of its contents (hence our ability to identify the poem), and it came to be known as Otho A.xii (in keeping with Cotton's system of placing his manuscripts in cabinets with the bust of a Roman emperor, listing the shelves as A, B, C, D, etc, and then counting the number of books along). The remains of the manuscript survive in the British Library under the full shelfmark of 'London, British Library, Cotton Otho A.xii'.
Yet having said all of this Maldon does not, in a sense, exist any
more. In 1731 Cotton's library at Ashburnham House was gutted by fire and many
manuscripts were damaged or lost altogether. In the inferno the folios containing
Maldon were completely destroyed. Yet all was not lost. Shortly before the fire
a careful transcript of the poem had been made (possibly by David Casley, the
then deputy keeper) and this did survive and now resides in the Bodleian library
('MS Oxford, Bodleian Library, Rawlinson B. 203'). In other words all of the
editions we have of Maldon (including the one presented in M&R) are based
on this copy of the original manuscript.