Can't be Custer, who was blond, didn't wear a beard, had a mustache and goatee which were closely trimmed for his last campaign, and carried Bulldogs, not a Colt. Also, his campaign hat was a tailor made hat, and his brother Tom and W. W. Cooke had similar hats-- the "scout" on the box top is wearing a different "Stetson" appearing hat. At the Little Big Horn, Custer wore a pair of leather breeches and a blue fireman's campaign shirt with yellow or white trim, purchased by many officers of the 7th Cavalry. His buckskin jacket was left with the pack train, and still exists today. Little Big Horn troopers would have worn the later sack coats, and sky blue trousers, most of which would have had the inseam reinforced with the white fabric from flour sacks. Walter Camp's notes on the Little Bighorn is probably one of the best first person resources for the campaign, and consists of personal recollections and evidence from both sides. That book and Randy Steffen's four volume series on the US Cavalry are probably the best sources you can find for uniforms and accoutrements of the cavalry on the plains. Don't get too hung up on the dates for implementation of uniform changes, as this was a tumultuous time in the history of the US Army, and for many years after the Civil War, earlier uniform styles were worn as existing stocks were exhausted. The Post Civil War years were a time of "belt tightening" and benign neglect for the Army, and it took a long time for new weapons, uniforms and equipment to reach troops at distant posts in the old west even if Congress grudgingly released the funding in the first place. So it's certainly feasible to have seen an 1868 uniform coat on an 1872 trooper.
VR, Russ