The Valjoux Cal. 23 VZ Is A Celebrated Hand-Winding Chronograph Movement In The Valjoux 72 Family Produced From 1916 Through 1974. It Was Replaced By The Faster-Beating Cal. 236. Implementing Willy Breitling’s Dual-Pusher Design With A Column Wheel, It Remains Desirable To This Day. The Design Of The Chronograph Works, Visible At The Back, Are Particularly Appealing.
The "VZ" Name Stands For "Valjoux Zähler", Meaning Simply "Valjoux Chronograph".
Source: Grail Watch Reference