Aren't they technically Atlas versions as they have 4 bowl clips rather than 2?Indeed, they have four clips rather than two, but the columns they are mounted on look as if they were installed in the 1980s, and Wessex Way and the roundabout that these are on was built and opened in the early 1970s I believe, a few years after the Atlas brand was dropped. Furthermore, I think Bournemouth Borough was more keen on GEC products when it was a part of Hampshire, and Thorn started to come along once it moved into Dorset, not that I'm saying Hampshire never used Thorn, because that statement would be untrue. I am also pretty sure that Wessex Way and this junction would have originally been SOX lit. Other sections of Wessex Way were lit with SOX until about 1990 when many of the original columns were replaced, with the exception of through this junction, where the original stepped tubular steel columns still exist and are fitted with Triumphs.
EDIT: Mike's 1972/3 catalogue shows the normal deep bowl variant having two clips but the shallow bowl variant having four clips. Perhaps the lower profile bowl was more flimsy due to its flatter shape, and Thorn thought it best to use four clips?