S&W has said that any of their .38 Special revolvers which have a model number marked on them are fine with .38 Special +P. Revolvers made for .357 Magnum are good with .38 Special +P and +P+.
You might like to review the following report of the effect that +P and +P+ ammunition has on the .38 Special and .357 revolver. Interesting article that addresses .38 Special and .357 revolvers use of +P and +P+ ammunition. The cylinder may be capable of withstanding the increased pressure; however, the added force will increase wear and reduce service life. Use +P and +P+ ammunition sparingly.