This is an example of a rare .22 caliber First Generation Single Action Army, one of 90 that were originally manufactured as .44 rimfire revolvers but were converted to .22 caliber before leaving the factory in 1888.
Many of these revolvers were subsequently altered or destroyed. This example is listed in the "Colt Peacemaker Encyclopedia Volume 2" as having been shipped to Kittredge on July 21, 1888, along with two other .22 rimfire revolvers. At first glance the revolver looks to be a standard .45 caliber revolver until the bore and hammer are looked at more closely. It is marked with "22 CAL" on the left front trigger guard shoulder over the original "44 CAL".