I believe the backstrap is left open, and the metal exposed to cut down on the size of the grips. Like how most J-frames from S&W come like this with "boot" grips, for max concealability.
I would have preferred grips like this on my 817, the frame is about the size of SP101, but weights only 21 ozs and has a 2" barrel. The 817 would be better served with a two-finger, smaller grip. I know some like the ability to have your pinky on the grip and the backstrap covered, but it would be nice if Taurus gave the option of "either/or".
But I got my Hogues, and they are better than the "ribber" grips in my opinion as they feel better, and are a little more compact.