I have an older S&W revolver build date is I think 1957. It has a 4" barrel & fixed sights. cal. is .44spl. . ser.#S142678. I've been told it is a model of 1950 M&P, also been told it's a model 21. Can anyone clear this up for me? Is this an early version of the thunder ranch .44spl.? This is a VERY nice revolver with about (I would guess) 95-98% bluing. Any help would be wonderful.