Not knowledgeable, but have the right book. “Precious Metals
Science and Technology” published by the International Precious metals
Institute " in an English translation from Japanese , copyright
1991, ISBN?. Expensive!
Pages 438-443. This is as hard as you would expect to get, as
annealed , in the K10- K9 series. Vickers hardness 120-130. tensile
strenghth at the peak and elongation at the minimum with respect to
copper concentration. Material can be age hardened at 300 C. Color
I will try and photo copy the respective pages and send off line.