OK, here is a cal 66 from 1907 ( not my watch).
There is nothing special to observe, only "quality extra". But this watch was awarded the "Bulletin de Marche" or chronometer certificate from the Astronomic Observatory in Neuchâtel, more than a century ago. Many hours were spent to assemble and regulate the watch, all by hand. There was a time, after the period of Jones, Seeland and Tschopp that IWC had no top watchmaker to regulate these extremely accurate mechanical time pieces. During that era the expert "regleurs" were traveling throughout Europe as "artists" of horology. These experts were hired or employed by Zenith, Omega, Vacheron Constatin, and many others, IWC hired them too, in order to deliver to the Astronomical Observatories their outmost finest timepieces for time testing. Every serious Swiss manufacturer needed these watches and their performances for advertising reasons.
Kind regards,
Adrian,
(alwaysiwc).