I haven't been able to find an exact date, but several sources say "a few made in .45 Colt" between 1875 and 1889.
From Chuck Hawkes
"Most 1875's had 7-1/2 inch round barrels, while a very few late production guns were supplied with 5-3/4 inch barrels. Calibers were .44 Remington, .44-40 and .45 Colt. .45 Colt revolvers had slightly longer cylinders to prevent inserting a .45 Colt cylinder in a .44 caliber frame."