On the AEC V8 thread, there is some discussion about the provenance of different types of Ergo sleeper cabs, specifically the factory “long” cab and the high roof version:
Further up this thread, this:
download/file.php?id=79013&mode=view
All of them appear to have different rear quarter panels. IIRC, there is mention somewhere on the forum of the method of assembly of the high roof cabs. I cannot find it, having skimmed through the first 24 pages
.