Strider,
Another thought to complicate things. If it is genuine, the tribal number probably won't do you much good because Congress has done a lot since 1921 in the area of its relations with Native Americans, especially the Indian Reorganization Act of 1934. It revamped tribal governments, who owned the land, and I would imagine, whatever cataloging system the USFG used to keep track of the various nations.
However, if you do find stuff prior to 1934, the number may help.
Good luck.