The easiest place to check is to call S&W parts or to look in parts on S&W's website. It is possible that they may have a couple in parts as potential replacements for previous runs they have done.

Personally, I would not want one on a J frame as it increases the weight without any appreciable increase in strengh. Heavier is less convenient to carry.

The weak point in any cylinder are the thin sections above the locking notches, and in cylinders having an odd number cartridges (J frames have 5 in 38 special/.357 Magnum), the locking notches are already in the beefiest part of the cylinder in between charge holes.

That is why the 7 shot L frames are arguable stronger than the 6 shot L frames.