This is what I found on Doxa site in FAQ section about Synchron:
Q. What is the relationship between DOXA and Synchron? Why do some vintage models say Synchron on the back?
A. After the crisis of the Swiss watch industry in the seventies, DOXA joined a group of Swiss companies to be able to resist the crisis the group was called SYNCHRON - it was in that time when the watches manufactured carried the logo DOXA by SYNCHRON. The following companies were part of the Synchron Group (Girard Perregaux, Eberhard &Co., DOXA, Borel, Cyma etc..)