apparently James, Virgil, and Newton Earp (Wyatt was too young) all served in the Union Army. Since they lived in/near Pella, Iowa I assumed it was an Iowa unit.....but Wikipedia lists Virgil Earp serving in the 83rd Illinois Infantry...
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From NPS Soldiers & Sailors System:

Earp, Virgil W. Co. C 83rd Illinois Infantry

Earp, James C. Co. F 17th Illinois Infantry

Earp, Newton J. Co. F 4th Iowa Cavalry

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Have always had a warm, fuzzy spot for the brothers Earp since my younger days, growing up in Arizona....

Hope this helps?

Kenneth Byrd