I went back in my ASME C&S connect to obtain the history on this code case. Apparently, Sumitomo developed a super 304H material and presented to ASME B&PV Materials Group in 1998. This code case was brought to ASME by representatives from Sumitomo.
Excerpt from the history file....
[The proposed Code Case is for a new austenitic stainless steel developed by Sumitomo Metals. Sumitomo refers to this new alloy as Super 304H. The steel is approximately 20% stronger than TP347H and reportedly exhibits superior oxidation/corrosion resistance equivalent to TP347HFG. Documentation submitted with the request for a Code Case showed that 14 jobs with a production total of more than 3000 tons have been fabricated by Japanese boilermakers between January 1989 and October 1998.
Sections I, II, IX, and the SG-SW (SC II) have all reviewed the material and concur with its use in a Code Case.]