There is an article online dated September 6, 2015, by the Pioneer Press titled, "
Gun used to kill bandit in famed 1876 raid returns to Northfield." Apparently there is also a watch given to him by one of the banks and a pistol he carried after the incident. You have to supply an e-mail address to get access to the whole article. The gun does not appear to be a cut-down musket but instead a breech-loading Smith Carbine.
NORTHFIELD, Minn. — The rifle used by Henry Wheeler, a pioneer physician, to kill a member of the James-Younger Gang and injure another during the Northfield Bank Robbery in 1876 now is on permanent display in Northfield.
www.twincities.com
There is a photographic portrait of Henry Wheeler seated and holding a fur hat in one of his hands at this link:
Portrait of Henry Wheeler Sr. seated in a chair wearing a suit. Arm resting on a small table, other hand holding a hat. Sepia-toned photograph with mark that appears to be from tape at the top.
contentdm.carleton.edu
There are a number of images on a flicker account including the carbine, Cole Younger's pistol and belt rig. It's a Moore Revolver, the belt is black leather army surplus with a rectangular eagle plate and black leather holster.
Smith Carbine at:
.32 cal. rim fire single action model 1860 Moore revolver at: