I confess to not knowing anything about Sigma! The Vénus 170 chronograph movement was very popular and was installed in many famous watches such as Heuer and Breitling. Sigma was probably a micro-brand (if you know different please let me know.).
 
Vénus officially came into being as a maker of ébauches (watch movements) in Moutier, Switzerland in 1924. They produced their first chronograph movement in 1933. The calibre 170 arrived in 1936. Interestingly Vénus sold the calibre 175 to China where it is housed in the famous Sea Gull ST 19. Vénus was absorbed into Ebauches SA, later ETA and Valjoux.
 
The watch for sale here is a fine example and is in great condition and has the famous 6/12 configuration. The small seconds is in a sunken sub-dial at 6 whilst the 30 minute accumulator is in a sunken sub-dial at 12. Blued hands. Depress the top pusher and the sweep seconds hand starts. Depress it again and the hand stops. Depress the bottom pusher and all hands reset to zero. Note that the sweep seconds hand resets slightly past 12. Both pushers operate smoothly and show few signs of wear. Signed Sigma in black.
 
The case measures 33.5mm in diameter (excluding the crown and pushers) so is on the small side and is more suitable for the smaller wrist. It has a robust, chunky look to it. It's made from stainless steel and has few signs of wear.
 
Inside is the Vénus 170 calibre movement. It's in good condition and keeps good time.
 
The watch is fitted with a sand coloured NATO style nylon strap.
 
An interesting piece of horological history.