Smith and Wesson rebound slide springs are closed and flat ground on each end. (17 total coils)
Aftermarket rebound springs are closed wound but not ground flat. You may also see rebound springs that have been cut to a shorter length.
Aftermarket mainsprings sometimes have a "rib" down the middle of the spring, and may have a distinctive bend that cants away from the hammer in the top 1/3 of the spring, below the stirrup claw.