Alas, AFIAK the Jubilee mounting bolt is unique to that model and has been out of production for several decades now. The bolt is longer than the DuoPar/Success/Challenger bolt but much shorter than the mounting bolts on most other derailleurs of that era. You may have to source another Jubilee to get that part, or have a machine shop fabricate a replacement. Be aware that you will also likely need the specific tabbed washer for your dropout style -- Campy or Huret type.