Potential frame rot?

It does appear to be a weld, but the metal in the rail behind it seems solid. Looks like it’s in a spot where someone might have put a jack or stand, so it might have been an attempt to repair some lifting damage.

But it does look like the crack continues, so it may be accident damage. For these old cars it doesn’t look like any kind of terrible rot, so my concern would be accident damage. If it’s just what’s in the picture I wouldn’t be worried about it, but I’d have to see the rest of the frame rails, floors etc for any other signs of an accident.

These cars are over 50 years old, none of them are perfect. I wouldn’t freak out if that was the only abnormality, but I would make sure it’s not a symptom of a larger problem.