Truth be known, the whole "extra cylinder" thing is a Pietta thing developed recently for marketability.
Not saying older Uberti and Pietta cylinders can't be swapped; only that the newer Pietta cylinders are "cloned" so a spare cylinder for a particular model will be a drop-in perfect fit on a like model.
Again, this is slick marketing on Pietta's part, as historically few if any soldiers or cowboys had an "extra cylinder" (Pale Rider-style) and if they did, it would have to be hand-fit to their gun.
But it sounds good to newbies, and sells a lot of extra cylinders.