Hi folk's, does anyone know what the square cutouts are for on the sides of the tool box that is fitted to vanderbuilt tenders?

Yes.

They are to allow shop workers to access the coil springs in the tender trucks.

oh, and Peter:

The replacement toolbox we had made was of welded construction and not riveted. And the fabricators took some liberties with the construction, simplifying the brackets that hold the tool box to the tender frame.

Of course, it would have been nice if they could have riveted the tool box together, but seeing what happens to the riveted boxes after 50+ years outside, maybe welded construction was the way to go?

If you can wait 50 more years, we can see how durable welded construction is, then you will know what to request from your fabricators....

Thanks Joe, your drawing is a bit different.. notice no long bar as a hinge like on 6069.. we hope to salvage the door for the new box and I believe we may go with the dome headed bolts in lew of rivets so that it still looks somewhat authentic