You can send the cylinders with the black powder frame and the black powder cylinder installed without any problem. Installation is the manufacturing of a firearm part, and that's perfectly legal for anyone to do. There's no prima-facia crime involved that the BATF would worry about (like shipping a short barrel and a semi-auto Uzi together in the same box, IE: Constructive Posession of a NFA Weapon), so... it's just what it is. A black powder replica (legal) and conversion cylinders (legal).
I am not sure the OP is doing anyone
any favors by highlighting this information for all to see though... Maybe Ms. Feinstein will read this and run to close another "loophole". Why don't you think before posting perhaps?