Mike from St. Pete pretty much said it.
The 39 series was popular but the coming of the 59, and the demand by police and civvies for more "firepower" led to the 39 family to lurk in the shadows. It wasn't till the compact 3913 that people recognized what a delightfully slim pistol the 39 series was, that and that accuracy beats firepower every time!
The 639 is a true beauty, wood grips on a stainless steel chassis...yeah I've drooled on a few myself!
its heavy meaning recoil is light even with hot loads, it will eat +P all day long, and take bad weather or sweat in stride as long as you wipe it down once a week.
The only downside I can think of is the double action trigger pull might be a bit tough, but since you shoot revolvers your ahead of the curve already. Other than that they are a nice reliable shooter and the accuracy ain't bad either.
Dale