Hi there, I really don't know where the parts image came from originally. Looking at it, this evening, I am having a job believe that it is for a P38. Just the image part, showing the rails, don't quite look like those on my P38.
I have sent an email to JG Wade on the off chance that they may still have one of the finishers in stock, but I have a recollection that I got the last one that they had, and it was in a very old package. My finisher clip thingy is positioned over the head of the front seat passenger, kind of on the side of the sunroof, not as shown on the diagram.
Failing that, I can only think that either a P38 dismantler, or see if it could be 3D plastic printed. I am getting the power steering pipe clips, that are under the radiator, 3D printed, although with a somewhat different design.
I do think that this is something that could be hard to find, unfortunately, and it would really bug me if it was missing.