[rhmc24]

Small flint pockrt pistols, opening of the frizzen was apparently a common hazard when drawing the gun from a pocket. They are seen wih latching devices with a pin that engages the base of the frizzen. A small knob behind the hammer operated it, usually served a dual purpose of engaging a notch on the back of the hammer as a dual safety.

See example:
http://www.collectorsfirearms.com/belgian-spring-dagger-flintlock-approximately-54-ah2980/