A French Napoleonic First Empire Waterloo-period M.1804 Pattern Back-plate.
The backplate of heavy plate with bold medial ridge and flanged base, studded with 9 brass rivets around the base and six along the neck (See Aries No. 4, 1969, Fig. 2d.), dark patina overall with patches of light pitting, generally good condition £500-£700

Almost certainly a souvenir collected after the Battle of Waterloo (see Waterloo Witnesses by Kristine Hughes for numerous mentions of British visitors to the battlefield in the days and weeks following the battle particularly trying to obtain these cuirasses, of which there were many at the time lying around on the battlefield).