It's not uncommon for pitting to occur under the grips on a pistol or revolver, particularly if it has been conceal carried extensively.

Moisture and sweat get trapped under the grips next to the steel, and if the grips are not removed on a regular basis to clean and oil the metal there, rust is inevitable. Rust is hygroscopic (attacks and holds water) so once you get a little rust, you soon have a lot of rust and a pit.

Nickel plating isn't as much protection under the stocks as you'd think as the contact points on the stocks, and any grit that has been trapped there, rub on the plating and over time.

Worse, as noted above, if you use a cleaning solvent with ammonia and do not wipe it off and then oil the surface, that ammonia will eat the nickel plating. And of course, if that solvent wicks under the edge of the stock it will stay there and eat away at the plating much faster than just the residue of the solvent on an exposed surface.

I only have one nickel plated revolver, but I don't use ammonia based cleaning solvents on it at all, and instead just clean it with CLP: