Eliminating the big crimps to the far right as well as the juggy top hold for the right hand would give it a 6c. Otherwise, as a sit start from the crimp as drawn on the photo and using all available holds must be around 6b.
If you start with the left hand up on the arete already, it's just two long moves and a hard mantel. Starting on the underclings only, as indicated, is much much harder (which would justify the + in the grade).
This annoying little low-ball has been tried by many. It looks a bit forgotten and neglected, like it would be too easy for the grade, but seriously, it's quite weird, height-dependant, and it takes some figuring out, if you don't know the beta in advance. After you get it, it can feel effortless...and that's what makes it so perfect.