Find a picture of the Dan Wesson Guardian Duce (Google or Bing). It has the rib like the Gold Cup.
Have you thought of adding a rib and then machining?
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)