This is from what must have been a very fine quality brigandine. Comprised of a single iron or steel lame of parallelogram form, gently clipped at the basal corners and top proper right. Along the top proper left edge is a row of five gilt brass rivets with flattish round heads under which is retained a section of cloth-of-gold over canvas fabric. Another set of widely spaced rivets is found on the face, just above mid-height. The plate could be from a backplate, or a column of plates next to those found at the armpit.
The same mark is found on the left plate of a brigandine breastplate from Xalkis (29.150.90.Q) in the Metropolitan Museum of Art, another left breastplate (29.151.90.FF) and backplate (29.150.90.M). A variant with a recessed outlined "B" appears on a right breastplate (29.150.90.T) and two backplates (29.150.90. C,D) there. When our piece was studied by Ian D.D. Eaves, formerly Keeper of Armour, Royal Armouries Museum, in the 1980s, it was suggested that it might have been related to the Metropolitan's left breastplate (29.150.90.FF) and backplate (29.150.90.M), as only these and ours have the same rounded, gilt brass rivets. The rivet arrangement with the isolated rivets can be found on some fragments, and in art, from about 1450 into the 16th century.
Guy F. Laking, "A Record of Armour and Arms. . . " 5 vols. (London: G Bell & Sons, Ltd., 1920-22), II: 191-200. F.M. Kelley, "Pre-Gothic Cuirasses of Plate," "Apollo" (1930), vol. 12: 37-43. Charles Ffoulkes, "Italian Armour from Chalcis," "Archaeologia" (1911), vol. 62: pp. 53; 390, fig 7 Ian Eaves, "On the remains of a Jack of Plate Excavated from Beeston Castle in Cheshire," "The Journal of the Arms and Armour Society," (September 1989), XIII: 99, note 6; 102, note 11.