The experts will be along shortly, but on fixed-sight S&Ws of that era the serial number is on the butt; the digits on the crane are an "assembly number." If you are correct on the manufacture date ('49-50) then it is called a "Military and Police." The model numbering system was implemented in 1957-1958 IIRC, at which time the "M&P" received the designation "Model 10." This assumes your revolver is chambered in .38 Special; the .38/200 nee .38 S&W was called the Model 11.