I've owned two, one for a Ruger Old Army and one for a Colt Blackpowder Arms Walker. The Ruger just dropped in. The Colt revolver had to be sent to R&D so that they could fit it. Took months and months. Now if you buy a Walker cylinder from Taylor's I think that you have to do the fitting.
I asked Taylor's about that at the last Shot Show. They said that most Colt cylinders did just drop in but not all.
Taylor's, incidentally, does not, as far as I know, sell all the cylinders that R&D makes. The Spencer cylinder, for example, is available elsewhere.
Of course R&D isn't the only company making these cylinders but they are the only brand that I have used.