looks like a square butt K frame from the pic, but as the prior poster said, hard to tell from one picture. There may be a marking somewhere on it that gives a better clue.
If you zoom in on the rear edge you can make out the transition from open back to closed so it's for a round butt. The square butt version is open down to near the bottom. Good find.
The square butt ones (with logo and notched on both sides) unfortunately weren't used for long and are a bit hard to come by. I have some I need to put in the classified section.