Don't know exactly, but suspect larger cylinder bolt, larger notches, and possibly stronger springs where needed. I found references to a "floating hand" as well. I'd suggest contacting Smith for specifics for your gun, which based on the -number, should fall into the improved series I would think.
I seem to recall reading that not all the mods were done to all calibers, specifically the .357s, but you know how reliable an old man's memory is!
