The design was a combination of the Argus and the Britannia. It had the Brit wings and tail shape, but the fuselage was an entirely different structure with tied floor beams. The Yukon was developed first, with two freight doors and the small Brit cockpit windows. All CL44s had the swing tail (an Avro invention) and large (Convair 880 design) cockpit windows. The 'plug' I think you refer to was when the Icelandics stretched some of the D4 models to produce the CL44-J, the largest passenger aircraft (nearly 200 pax) to operate on the Atlantic at that time. It must the the only freighter to have been converted to pax! The Guppy was designed by Jack Conroy, I think for NASA. It was built in Santa Barbara by cutting a standard D4 fuselage down to floor level and fitting the large fuselage frames, complete with extra attachments for the swingtail. Does anyone out there know about the 'cold war' secondary role of the CL44 design as a military freighter? . It could carry ICBMs and its 6 generators could produce 300kva.