Cal. 20 CH is a 13 ligne chronograph movement produced by Minerva. It is officially called “Chonographe-Compteur 13/20 CH” on company data sheets. It was available as a monopusher (with 5 columns) or later as a a 2-pusher (with 6 columns). It was a compact movement suitable for use in wristwatches, though the small Minerva company never became a dominant player in the chronograph movement space.
Production began in the 1920s (perhaps 1923) and continued for decades.
The basic design of Cal. 20 CH was revised and reissued in the 2000s after the acquisition of Minerva by Montblanc.