The coating is a plated layer of hard chrome.
This is often called a "flash plated" coating.
Most any gun re-finisher service that does hard chrome can strip the old coating off and apply a new one.
Depending on the plater, you can have the original satin pearl-gray as used by S&W or a satin stainless look that more closely matches the stainless of the gun.
Like DEFARIS said it is hard chrome finish applied to surface of carbon steel hammer & trigger to resemble stainless. The earliest SS models had solid SS hammer/trigger but were plagued by galling at their contact points and early on S&W went to the plated carbon steel which cured the problem.
