Forging Stock and Forgings of Aluminium- Zinc-Magnesium-Copper- Manganese Alloy (Heat Treated at a Ruling Thickness of Not More Than 75 mm) (Solution Treated, Step Quenched and Precipitation Treated for Low Internal Stress) (Zn 5.7, Mg 2.5, Cu 0.5, Mn 0.5) (06.71)