I used this part to repair a Triplett VOM meter movement protection circuit.  It is a NPN - PNP pair arranged so that if one is experiences a trigger into conduction the other maintains the trigger current until the primary current (anode to cathode) is removed (or device is "untriggered").  This unique feature of the device lets it be triggered into conduction (latch) then it can also be triggered off with either polarity pulse (unlatch).  One microamp will trigger ON, part can handle 200mA and 65 Volts. Triggering OFF requires a current near the primary A-K current.  TO 72 has all 4 leads gold plated.