Can anybody provide any more information than I have already? All I can find out that it is a high strength cupro-nickel, (probably) a forging spec, and comes under NES 835.
HIDURON® 191 is the mid-range high strength alloy and has been used extensively in naval vessels for the Royal Navy and US Navy for critical components in weapon handling systems and control elements for submarine bow plane assemblies. Typical features include:
Excellent general corrosion resistance in marine environments
High strength (UTS 725 N/mm2: 0.2% Proof Stress 430 N/mm2)
Resistance to hydrogen embrittlement
Good anti-fouling resistance #