[Graham]

I've never looked into this cars past before but as you say there's different wheel arch types, at least three distinct types for the CSL.

There's dozens of photos of that very car listed under BMW 3.0 CSL Group 2 Works on ultimatecarpage.com

I had another quick look on line, and genearlly speaking I would say the BMW 3.0CSL Group 2 could have extended front & rear Wheel arches and be ligitimate, but the main difference "seems" to be that the Group 5 had broader rear wheel arches with engine intake vents fitted into the rear wheel arches , presumably because the Gr5 version had Turbo assisted motors ??