Let me add that if you are interested in the revolvers to use for defensive purposes, you might want to consider the Model 14. The K38 Combat Masterpiece is not rated for +P pressure ammo but the Model 14 is. The higher pressure loadings allow for some additional velocity and potentially better penetration/expansion of bullets. If this is a range gun, then I would also go with the K38.
I suspect that the primary reason S&W won't rate pre-model number K-Frames for +P is because of the variances in the quality of their metallurgy before WWII and because model numbers are an easy point at which to draw the line. That being said, the K-38 Masterpiece was not introduced until after
WWII so all of that particular model will have reasonably modern metallurgy. Because of this, I would place a pre-Model 14 K-38 Masterpiece in the same category as a pre-1999 steel J-Frame: they're OK with limited use of +P ammo so long as they're not fed a steady diet of it.