On a 106-P movement

 

"Mouvement pincé"
extension B: cam disc regulator

http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&0&2uswk&Zenith_106_P

 

What is that exactly?

 

That’s not what it is

 

A/ looks like upper pivot for the centre sweep pinion underneath its removable bib.
B/ looks like retainer spring to hold the centre sweep pinion bib in place.
C/ tension spring (brake) for the centre sweep seconds pinion to prevent “jerking” of sweep seconds hand.

 

I agree those are the parts for the center seconds, I found these photos of calibers 106-P-5 and 106-P-50, yours looks like the former, the -50 version looks like an improvement with a cock rather than a spring to support the pinion.

 

From what I could find, there is no 106-P-5 just 50. I suppose it’s possible they’re marked incorrectly.