No, it is not something cobbled together by an enthusiastic photographer/machinist. It is a single unit. If you have looked carefully, there are no markings of any sort - aperture, focus, etc. Not even the Pentax name. All these would have been present if it was a modified item.
I have not seen this specific barrel for anything other than the 200mm Imagon, and over the past four years, I've sold around a dozen of them, some including the lens, disks and hood.
Kumar