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.