I did not realize that the 640 ND was all that special. I bought mine quite a while ago, and I consider the double action trigger pull to be the best of any of the J-frames I own.
This one has Herrett's "Shooting Star" stocks made many years ago to fit my hand perfectly - originally on a Model 60.
And no, I can't be talked into parting with this stout, reliable and slick little gun!
John