1967 door lock

Could someone tell me what it is that actually lockes the door lock rod up and down? When you pull up on the rod it snaps unlocked and vise-versa...on mine the rod free falls down and will not stay up. All the rods are connected inside the door, but i cannot tell what should allow that rod to lock up/down. i was wondering it maybe it could be the lock cylinder?
 
back rod goes up to the lock knob. front rod goes to the door handle. Edit, I am saying this from memory. I will have to go out later and see if I was correct.

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This contraption holds the lock rod in the up or down position.