The historical accuracy/probability of having several spare cylinders aside, I think the filmmaker's intent was to show an alternative to typical SAA-type loading. Obviously, he was successful -- we're still dissecting the scene 25-years later.
Discussing Eastwood's films, non-shooters will often comment on Preacher's reloading sequences -- they recognize there is something different happening, but didn't have a clue about the particulars. Until seeing the OP's pic, I'd always assumed the spare wheels were of the percussion variety. Cap or cartridge, it doesn't change my appreciation of the film.