Pistol Grip spring lancet
Unusual pistol grip spring lancet c1850.  Item is of unknown origin but was purchased from a dealer in Australia.  A similar example is present in Appels book of instruments from the Museum of Natural History collection.  
The red arrow indicates the point of attachment for a cocking-bar.  The extreme tension on the mechanism requires a long lever arm to cock the mechanism.  The green arrow is pointing at the release button.  This image shows the flip-out lever on the base of the unit for improved grip.
The depth gauge has a ratchet mechanism to hold the end plate at the desired depth.
An unfilled screw hole at the top of the device would place pressure on the edge of the blade, possibly to keep the blade extended for cleaning.