Hermann

I am fully aware of various anomalies in the Heuers of that period including French production by Monnin of various 7765 powered models and I also own a rare 3006 (Vjx 7734) that was considered to be a 'franken' until Stewart Morley found it in the 1979 German catalogue about ten years ago.

I also have a rare fume Daytona with an odd model engraving error on the caseback using a Verona reference (one digit wrong) even though the Verona uses a different type of caseback to the Daytona. There are other examples : the 'Economy' Monza Cal 12 is often seen with a 150.501 caseback which relates to its Cal 15 cousin even though it has a different reference 110.501.

That said, The other examples of the 13 1 that you have found with a caseback consistent with yours all appear to have been heavily refinished on the caseback without the 'Automatic' script. There are definitely at least two types of 13 1 font / caseback engraving visible in images found on Google but no others I can find marked "Automatic" even when the font / engraving looks similar to yours.

How that caseback marked as "Automatic" came to be on there is a mystery. That said I would personally agree with Mathis and consider it highly unlikely to have left the Heuer (or outside sub contractor's) factory with that caseback fitted.

I would simply enjoy the watch - they are nice pieces with robust movements and the PVD case looks to be in good order.