It is common to most target-sighted S&W's made after they also dropped the P/R features. On the older guns, the frame was grooved from under the rear sight body, the sight body was grooved and the barrel's top rib was grooved, all the same way. It looks nice, uninterrupted grooving. P/R was dropped to save some machining steps and their costs, and I guess the lack of grooving on the frame's topstrap was just that much more machine time eliminated.

I thought this was kind of tacky. Although it is just cosmetic, I am surprised more people haven't noticed it.