Okay, the 1851 is a 36 caliber revolver, as built by the original maker. (Yes, I am aware of the very few 40 caliber revolvers of this model, I am refering to the "standard" Model 1851.) The same maker, used the same frame and cut a ledge in the frame. Using a higher grade of steel for the cylinder (silver steel, I believe it was called), the origianl maker enlarged the front of the cylinder to allow chambers of 45 caliber to be cut. The enlarged cylinder will not fit on an unmodified 1851 frame.
The replicas may not always be true to the originals.