Rare form spring lancet marked with the Simpson mark and the 55 Strand address. The address marks the piece to post 1863. The instrument is armed by pulling the handle back to a half point that allows depth adjustment with the turn screw on the side. After the depth has been set the arm is pulled all the way back rotating the blade into an armed position. The handle drops back into the starting position and is released by depressing the handle while holding the instrument with a "pen" grip.
The entire lower plate moves up and down in response to the knurled knob on the side, thus adjusting the cut depth.
The blade functions in a swinging fashion similar to a cats claw unlike most systems that operate on an inline fire system.