A similar pocket watch is described in the book of Zenith Roessler on page 81. Serial number 2658508, 1925, calibre 17 E.P.V.S., 16 stones.
I also have a niello silver Zenith pocket watch marked on the cuvette "Grand Prix Paris 1900". My watch has a chronograph function with a register for minutes along with telling time. I will try to get it from the bank and take some pictures. JohnCote
Here are the pictures of the aforementioned clock. The regulator pointer has its own block for the better regulating of the balance spring.