I have had two stainless "J" frames with finish issues.
1. Model 638 with black marks on the frame under the grips.
a. contacted S&W Customer Service who had me send them
the weapon. Black marks turned out to be cracks.
Smith and Wesson replaced the weapon with a new one.

2. Model 642 with black pencil tip spots on the left frame.
a. contacted S&W Customer Service. weapon sent in. Weapon replaced with a new weapon. S&W would not advise me what the black spots were.