I don't have the book, I just found the quote. I have to say, that the comments by Mr. Weinbrenner argue strongly against the practice of swapping cylinders. But the comment in that book, and some of the stories of Mosbys men argue against him. Could be that an extra was carried in a cartridge pouch. Or some other issue gear. And, in the hundred years or so before he started collecting extra cylinders could have gone astray...look at how few of the revolvers are around. Overall, a puzzlement.