This 1961 “Timers and Chronographs” catalog is the first of the orange catalogs, that would be produced through 1965/66.

On page 24 of the catalog, we see the first designation of chronographs as “Economic” models, with the Reference 3641 and 3645 models.  The accompanying price list shows that the nickel-chrome Reference 3641 sold for TBD and the gold-plated Reference 3645 sold for TBD, compared with the stainless steel models that ranged between TBD and TBD.