this should help:
"'In 1952, Smith & Wesson applied the concept of the Safety Hammerless to the J-frame Chief’s Special and got the Centennial. The gun was named in honor of the company’s 100th birthday. In 1957, when the switch was made from named models to numbered models, the Centennial became the Model 40. Also in 1952, an Airweight Centennial was introduced which became the Model 42. Some 37 specimens of the Model 42 were built with aluminum alloy cylinders, but the rest had steel cylinders. These two models were produced from 1952 to 1974.
In 1990, the Centennial was re-introduced but in stainless steel and without the grip safety as the Model 640. It was a .38 Special. In 1996 the 640-1 in .357 Magnum was offered. Airweight versions, Models 442 (blued) and 642 (stainless) were also brought to market. As noted earlier, the Model 642 has been enormously successful."'
edited to add: also, i find this site useful as i'm a j-frame kind of person: http://world.guns.ru/handguns/hg105-e.htm