Originally Posted by CliffordK

Is that the downtube that is broken?

I've seen a few broken seat tubes, but I think that is the first downtube that is broken like that.

For a seat tube, I believe one could add a sleeve insert, and it could be welded or brazed to make a very strong joint.

The downtube naturally has less internal access. One could still make a reinforcement ring on the outside of the tube,and braze it for a visible repair, but it might not look bad, especially if you make it look like it is supposed to be part of the shifter block.

How is the shifter block attached? Brazed or welded?

You may be able to TIG weld the repair, although if the shifter block is brazed, you would risk contaminating the weld with brass, unless you're very careful with removing the block and cleaning the joint, welding then putting it back together (thus brazing an external ring onto the joint might be easier).

One of the issues is that some of the older alloys were supposed to lose some strength with TIG welding. You'll have to decide if that is acceptable. Also, with double butted tubing, the mid-tube thicknesses are paper thin making welding more difficult and weaker.

I'd encourage you to talk to a local bicycle framebuilder, or if you can't find one, even a welding shop (that can both braze and TIG weld) before you make a final decision on the frame.

Those drilled holes look odd. What was in them? If welding, however, they would be easy to fill.