During the CW there were no Colt cartridge conversions, you're right. From an armorer's perspective, I can certainly see why they use cartridge firing blanks for movies, they are just more reliable than the cap and ball...a misfire means the scene has to be shot again.
Now, Tuco has a 51 Navy that will change.....in some scenes, the long cartridge cylinder is evident, then in others, there is a percussion cylinder in it's place.
Angel Eyes has a New Model Remington Army, commonly called a "'58", and lord only knows why he has so many cartridges, looks, yeah, it looks good.
The really neat gun is the one used by Blonde.....look close, you'll see the frame is an 1872 open top, while the barrel is a '51. You can see the loading gate and the bridged area above the hammer, also the hammer is that of the OT....kinda neat....yes, DVD's are great.
Now, you want to talk conversions in CW era movies? Watch Outlaw Josey Whales closely.....when Josey digs his pistol out from the fire ruins of his Missouri home, he pulls out an 1860 Army, with a shortened barrel, no ejector or rammer and the conversion ring and cylinder and hammer set up are from a later Richards-Mason Army conversion.....also, you'll spot a lot of conversions in that one all over the place, from the two gents in the Trading post, to the one Chief Dan George uses....a Richards conversion with a shortened barrel, complete with a proper Richards ejector, but no ejector rod!
Still these are great movies to watch and they are entertaining, which is what they are supposed to be.
Probably the best film for showing percussion guns in their natural state is Quigley down under.....several scenes you'll see caps on the cones of the cylinders.....even a reloading scene complete with flask!
Next best one has to be Ride with the Devil, only reaason it falls behind Quigley is the main character uses "1858" Remingtons with brass frames! Other than that it's great.....they used quite a few cylinder from R&D in this one and the firing pins were painted to 'look' like caps on cones!