Alain,
Are you sure the car never was used on the track before you bought it?
My DD tube developed a crack after 38000km, 2.5years and regular track use. At that moment it was only barely visible in the paint ... Phillipe luckly detected it while replacing the dampers. I guess that after extra time, curbstones, etc. it would have ended up like yours.
It was rewelded and I now developed the good habit of checking it on a regular basis, including before and after every track day (not only the DD tube, also other stresspoints!!).