first the easy part, model 55A Target and the model 50 Sidewinder are the same except the model 50 Sidewinder has a rod ejector (like colt single action) and slightly different shaped grips (more of the plow handle shape of the colt single action), the grips from the model 55A will interchange with the Model 50 and vice-versa. the model 55A does not have any mean of ejecting the spent cartridge you must use something small enough to fit in the cylinder and punch them out or remove the cylinder and use the center pin. at one time in the 1970's iver johnson furnished an lshaped metal rod for this purpose which they called the johnson jector.
if your model 50 sidewinder is chambered for 8 rounds and has fixed sights it is not suitable for a 22 magnum cylinder. when the Model 50 sidewinder was first offered as a combo model the chambering was changed to 6 rounds and the bore diameter was increase to 22 magnum size (slightly larger that 22 long rifle) which is why most combo revolver (no matter who makes them) are more accurate with the 22 mag cartridge.
model 55/55A introduced in 1955
model 50 introduced in 1961
model 50 combo introduced in 1974
by 1976/1977 only the combo model was offered.