lock piece
/ˈlɒk piːs/noun
1 Mining. A prop to support the roof of a working. Now historical and rare.
2A piece of a mechanism or object for preventing the movement or displacement of something.
3In a firearm: a small projection for the attachment of a lock. Now historical.
Origin
Late 18th century; earliest use found in William Pryce (d. 1790), surgeon, mineralogist, and antiquary. From lock + piece.