Pulling from the Standard Catalog of Smith and Wesson...
Model 18's were produced from 1949-1985, with the pre-'57 models called the K-22 Combat Masterpiece.
Some model 18 revolvers were made in stainless steel. A few were also made with a nickel finish. A very few were made in stainless but stamped as model 65 (all but one is believed to have been destroyed.)
The nickel finish was available, but not listed in the catalog. The 18 (with no dash) appears to place the date around 1957-59.
Value? Well, the Standard Catalog puts a "as new-in box" model 18 at about $4700 with a nickeled version at about 50% more. No box, in excellent condition is closer to $425 with 50% more for the nickel.
Let me know if there is anything else I can try to find for you.
