I remember talking to Bob Simpson ( www.simpsonltd.com ) and asking many years ago about this front night sight that screwed onto the PP and PPK front sights. That's why the older guns have the little undercut at the rear of the front sight and the little indent in the front.
He said that people thought they were "sight protectors" and took them off and threw them away! He said they're also very, very rare and very, very expensive as collectors items.
I wish Meprolight or Trijicon or XS would make one again today. It would be pretty easy for a hobbyist to put the dimple and then undercut on a modern PPK themselves. The front sight could be made pretty big to match up with a Novak style snag free add on rear sight!