The action you are likely refering to is a gun made to Bentleys patent #17037 of 1884 it refers to a curved slot in the lock plate, in which the head of the hammer, shaped like a letter "C" swings. Its purpose was to act as a visual safety to show the position of the hammers.It was used on various actions by gun makers of the era. Guns by Turner are often used to illustrate this device.

Roy Hebbes