I emailed Bison Concrete this morning...thanks Tim... and heard back from a very helpful surveyor who can confirm that the Bison logo in the slab is the same one used by Bison Concrete in the 1960's. Bison Concrete now use a running Bison with outstretched tail and he pointed out the Whitefriars glass slab bison faces the other direction. I have suggested that a positive impression on the back would produce a negative when viewed from the front which might explain the difference in the direction the bison faces. I have also suggested the slab is actually an ashtray.... not unlike those large ones one we used to see in pubs;a simply depression in the middle so the tile curves down from the edges forming a slight well. Hopefully you can see this in the pic here. One can imagine they were produced in a limited edition for Bison by Whitefriars to grace their executive offices or canteen... or maybe as gifts and certainly between 1959-1970 when Arctic Blue was in use. 1960's would seem right. Once again many thanks Tim & Christine.