Marr's Lock

Invented about 1836 this lock (double chambers) is constructed at right angles through 2 studs in the main bolt. If the levers are over lifted, a small hook on the upper end of the small bolt is forced onto a stump fixed in the pate of the lock by a spring which retains it.
To release the small bolt the main bolt has to be shot a second time, in the same manner as Chubb's to release the detector.