Looks like one of the clips that hold the door operating rod to the door latch, or the lock rod to the door latch. The operating rods have a 90 degree bend on the end where they go thru a hole in the latch assembly. You put the part with the hole behind the hole in the latch, slip the operating rod thru the latch and the hole on this clip, then snap the compression clips over the rod and it keeps the rod from falling out of th hole. If it wasn't just a loose one that was dropped into your door 43 years ago, either your door lock, or inside door handle is going to quit working one of these days.
