I notice the talk about the rusty tanks.  I have taken off a few and noticed that the welded on ID plate that tells all the info about the tank, has collected gobs of rust under it through the years, making the thickness of the plate of the body half or less.  
You can get the plate off by grinding off the welds at the corners.  Only then you can see the huge damage the incubated rust has done, beyond imagination.  The wall thickness is paper thin.  
The new tanks now come with the same plate welded on, all around.  Lesson learned slowly.  
So, maybe your tank is not 'illegal' but it is finished if the rust has done its damage behind the plate.  

Zoltan