[Arran]

To repair that you will need to figure out what species of veneer it is. Contrary to what some may think they did use more then walnut, and they used more then one species and cut of walnut to get the look they wanted. The species of veneer used on that band was some form of mahogany I think, but it's hard to know for sure with the original finish still on it, it may also be Asia wood or Australian (Queensland) walnut, one problem you may have with patching it is that it was cut on the diagonal and book matched with the other side. The last one is a tropical tree and isn't really a walnut species at all. The vertical grained veneer next to that section is zebrano or zebra wood, it has alternating dark and light bands.
Regards
Arran