Some nuts Ive seen have the grade stamped into them.......the austenitic stainless steels are generally sold as suitable for marine use /salt water......the magnetic stainless steels are susceptable to rusting ,or at least rust staining.......
I would stay away from Beryllium, it is very toxic.
Since you mention this I need some 4" 6061 round bar? PM me if this is possible!You should've check here ! My scrap pile is endless .