Broadsword of the “Castillon” group
The diamond-shaped cross-section of this sword was developed in response to plate armor: swords like this could penetrate the gaps in a suit of armor and pierce the protective mail armor underneath.
This sword is one of the most intact examples from a highly important cache of swords excavated from in the River Dordogne in France. These swords were lost on July 17, 1453, by the retreating English forces at Castillon, the final battle of the Hundred Years' War.
Cf. PMA 2010-32-1. The Castillon group is discussed by Oakeshott in Blankwaffen (1980).