Have you taken the grips off yet? With that serial number, it probably is an early coil spring variant, but again with S&W "never say never" and it might be a late leaf spring model. I hope Hondo44 comes on soon and gives you the full story on it. BTW, this late with factory nickel (as can be determined by the N under the barrel) is a little less common. The 571620 number is the serial number and the other number is an assembly number to keep parts together after "soft fitting." After it left the factory it ceased to have meaning except perhaps to tell whether there has been some parts swapping... or in your gun's case not!

Pictures would be nice (we love pictures here) and a barrel measurement (from the cylinder face to the muzzle) would be questions some of us would ask.

Regards,
Froggie