2.25" with large metal rim on reverse holding fixed horizontal bar pin. There is a back paper at the center of the reverse but it is tanned and the text is no long there. Button includes large serial number "19" for use by an employee or participant. The Indian Congress ran August 4 to August 31, 1898 in conjunction with the Expo. The Congress was the largest gathering of American Indian Tribes of its kind to that date. Over 500 members of 35 different tribes attended including Apache chief Geronimo. Celluloid is tanned with light stain on lower right and surface has many small raised bumps noticeable to the touch. Still with full gloss. Only example we've seen.