Cozzens "Eyewitness to the Indian Wars" Vol.3 devotes a section to the Beecher's Island fight.Forsyth does mention Harry Davenport,there are 12 accounts of the fight,been awhile since I read them so not sure if he shows up in any others.He does place Davenport and others in the group as having served in "either the Regular Army or the United States or Confederate volunteers.
Interestingly Forsyth list what each man was equiped with," Our equipment was simple:a blanket apiece,a saddle and bridle,a lariat and picket pin,a canteen,a haversack,butcher knife,tin plate,and tin cup.A Spencer repeating rifle,a Colt's revolver,army size,and 140 rounds of rifle and 30 rounds of revolver ammunition per man-this carried on the person."