First of all, I assume we are talking about a target gun - not a fixed sight gun... ? Some .32 Magnums were fixed-sight models.
Several years ago .32 Magnum J-frame cylinders and barrels could still be purchased (from Brownells, and probably from the factory). Maybe someone who wanted a Model 631 and couldn't find it did a conversion? It would be a nice way to go, IMO, since you would end up with a proper square-butt frame rather than the silly round-butt frame that S&W used.
Check the frame, hammer, and firing pin bushing. If the gun is converted from a Model 63 frame, the frame would be drilled for the retaining pin for the rimfire firing pin. Might be a nice little shooter if you are interested in it. If not drilled for the retaining pin, then the gun is probably an original 631 that didn't get marked properly.