As a general rule S&W revolvers that have a 3 digit serial number starting with a 6 are made from stainless steel. There is a logic to the numbering:
610 (10 mm)
6
17 (.22 LR - like a model
17 in stainless steel)
6
24 (.44 special - like a model
24 in stainless steel)
625 (.45 ACP and .45 Colt - like a model
25 in stainless steel)
6
27 (.357 magnum - like a model
27 in stainless steel)
6
29 (.44 magnum - like a model
29 in stainless steel)
There are many other examples of a three digit stainless model starting with a 6 having a carbon steel (blue or nickel) equivalent in a two digit model with the same last two numbers of the stainless version.
There are some exceptions:
Models 64, 66, and 67 are stainless versions of carbon steel models 10, 19, and 15 respectively. Go figure.
Blue and nickel revolver are made with carbon steel, a significantly different and harder material than stainless steel.
Here's an interesting link to a post from a S&W Forum member regarding stainless versus carbon steel:
http://smith-wessonforum.com/s-w-hand-ejectors-1896-1961/115883-engineers-take-s-w.html