Recently, I have been plugging the holes in my S&W metal framed automatics collection. I came across a like-new immaculate conditioned 439 at a gunshow this weekend. The grips and finish on the frame and slide looked like this was a safe queen. Also at the same gunshow was a model 639 in excellent condition. The difference between the two pistols in terms of dollars asked was on the order of 400-bucks on the side of the 639. Why is the 639 so much more expensive than the near perfect 439?