Ashmolean Museum in Oxford. The third is in a private collection. My example is ex Dirk Kennis collection. The exact findspot regrettably is unknown, but it was found in East-Anglia.
My example is of the better quality than the examples of the Ashmolean Museum.
Executed in the Mammen style, it can be dated to the mid to late 10th century.
See for reference:
Viking Identities: Scandinavian Jewellry in England by Jane F. Kershaw, p. 116, 117 and 118 (Mammen-style brooches).
'Unique for Scandinavian-style ornamental metalwork in England, the brooches depict a Mammen-style bird.... the bird has an arched neck and forward-bent head with round eye and open jaw, together with a double-contoured head lappet; this arhes downwards into interlace with its body and wings before terminating in an open-ended tendril. The wings, represented by a pair of double tendrils, extend diagonally downwards to the bottom left corner and pass behind a similarly constructed tail, extended diagonally upwards. The bird's raised leg, with bent foot, is also visible. This treatment, particularly the movement and open-ended tendril terminal of the head lappet and the use of the tendril ornament to depict the bird's wings and tail, identify the bird with the Mammen style.
Yet whereas the brooch motif is influenced by Scandinavian art styles, its rectangular shape is not Scandinavian in origin. The brooch shape would instead appear to relate to to rectangular and suare brooches found predominantly in northern Germany and the Netherlands: so-called Rechteckfibeln.
Square-headed brooches are dated on the Continent to the 10th century, a date wich accords with the appaerance of the Mammen style in Scandinavia'
The best reference until now, I have found in the database of the Portable Antiques Scheme to a similar Anglo-Scandinavian bronze mount.
Also see the Viking-Age Scandinavian art styles and their appearance in the British Isles Part 2:
Thomas Kamphuis, July 30th 2015 (the image beneatch shows the brooch in its real proportions).