Please forgive me not very P.C. I know but, I seem to remember there where at least 2 'Slave Rings' set into the pavement in Liverpool. One of them was at the Junction of Moorfields and Tithebarn St, the other was in the vacinity of Stanley St, I think I remember them being there as late as the early 80's as I worked in Old Hall St and passed them many times.
As I said at the begining forgive me for touching on an emotive subject