There's two different types of Legnano binders. I would think the roadster style bike would take the "split wedge" style? The Roma shown in your second photo takes the "scalloped bolt" style.
Does your seat-tube have a slot in it running below the bolt hole?
I can show you a source for the scalloped type, but not wedge style which I think you need. It would be best to post photos of the actual frame you are working on to be sure.