I have an old S&W .38 Ser. Num. 658xxx
and as best I can read it...
38(G or 6) &W Special CTG.
You have a .38 Military and Police Model of 1905 - 4th Change, also sometimes called the .38 Hand Ejector M&P, or just the .38 M&P. It was made between 1915 (SN 241704) and 1942 (SN 700000), but obviously your SN places it more toward the end of the range, so likely late 30s, early 40s.
This gun was one of the earliest models built on the mid-size or "K" frame, which came to be the workhorse of the S&W line for police work or personal defense. With some modifications, it eventually became the Model 10, the basic .38 Special 6-shot fixed-sight revolver.
S&W made a lot of these, so prices aren't high except for perfect specimens. In good condition, somewhere around $300 or so - excellent condition, maybe double that.
Oh, that character you can't quite make out on the barrel should be an "S" (as in "S&W"), and the CTG just simply stands for "cartridge."