From what I found, C. L. Chester also did steorview photos for Keystone & Underwood.
In this photo, the rifles are long Krags and the soldiers got tan Mills belts with individual cartridge loops. The best guys to view are the two sergeants shown in the close up photo. They also seem to have a large belt buckle. This all seems correct for the Spanish American War. The soldiers also seem to have leggings that come up almost to the knee. You really need a 20x loupe to see some of the details in the photo.
source: https://www.globalsecurity.org/military ... tia-mo.htm
Missouri Militia Regiments - Spanish American War
First Missouri Infantry.-Official title, First Regiment, Missouri Volunteer Infantry. Headquarters, Jefferson Barracks, Missouri. Organized (from the First Regiment, Infantry, National Guard of Missouri) and mustered in for war with Spain, May 13, 1898. Strength in officers and men for war with Spain not reported. Name of officer commanding during war operations not reported. Stationed during war: (1) Camp George H.Thomas, Georgia; (2) Jefferson Barracks, Missouri; (3) Armory. St. Louis. Casualties during campaign not reported. The regiment was mustered out at Jefferson Barracks, Oct. 31,1898.