This Union (ex Chubb) 3M51 sashlock is a 5 detainer mortice lock with a hook bolt designed for timber doors with a thickness of up to 50mm.
These non-handed detainer locks have a hook bolt and a projecting stump on the forend that engages into the locking plate to prevent the door being lifted from the frame in the locked position
These locks are also force resistant, anti-saw and anti-pick and are generally used on sliding doors requiring an automatic closing action.