Ah-ha! A picture of the murder stone at last. It's quite nice innit.
http://www.themodernantiquarian.com/post/20697
Myself and Anthony Weir have a small unofficial classification of 'shouldered stone' for these.
stones such as
http://www.themodernantiquarian.com/post/11981
&
http://www.themodernantiquarian.com/post/3791
are quite common. The Murder Stone is a lot smaller and doesn't have quite the same pronounciation of the shoulder but I was instantly reminded of Tamnaharry.