I am not sure that any of these is actually an official grade listed in SS 14. 2906 is almost certainly referring to Sandvik SAF 2906 (UNS S32906, no EN designation), which is a super duplex stainless steel, used for offshore applications. Is there a chance that 2904 is really 2304? If so, then this is referring to Sandvik SAF 2304, which is a low alloy duplex stainless steel. The EN designation is X2CrNiN23-4 (1.4362) according to EN 10088-2, -3