I had two single six convertables for a while and it didn't matter which cylinder out of the 4 of them you put in either of the two guns. I also have a 44 mag super Blackhawk and picked up a 4440 cylinder for it and it dropped right in and worked fine. Rugers are hardly precisely made guns and I doubt theres a single bit of hand fitting at the factory.
Ruger won't sell you one without sending in the gun so it can be "fitted". But there's a fair number of them floating around that got separated from the original combo guns - keep checking the auction sites and gun shows.