Hello Hermann,
Happy New Year!
I have one and it has the same markings on the caseback as the examples that are on “On The Dash”.
Personally I suspect your caseback may have been from a different watch model and gotten modified but not 100% sure.
Additionally if you pay close attention to the markings and the writing on the back of yours vs the others, the others have (13 1) and (SHOCKPROTECTED) vs (13/1) and (SHOCK RESISTANT).
Could yours have come from the factory with that caseback?
Maybe, as I am defenitely not the resident Heuer expert.
But I will say the 13/1 font style on yours does not look quite like any font styles I have seen on other vintage Heuers.
It kind of looks more modern to me.
Maybe someone else will have a lot more insight into this particular model.
Regards, Keith

: Happy New Year to Everybody!

: Please have a look at this Ref. 13-1.

: Since we all know the Ref. 13-1 uses a Valjoux 7765 hand winding
: movement and not an automatic movement, I was a bit surprised to
: see the word „Automatic“ on the caseback.

: Could it be possible that this was an error made by Heuer or the
: case manufacturer?

: The engravings on the caseback, the marks of use all over the
: watch, shows clearly that the reference number was engraved at
: the same time as the other writing and not added later.

: So what do you all think about this?

: I am looking forward to read your thoughts.

: Thank you very much for your help.

: Greetings,

: Hermann

:

:

:

:

:

:

:

: