conversion work was undertaken by on mark engineering. the pregnant guppy (registered n1024v) was built from an ex-pan am airframe with a five-m section from an ex-british overseas airways corporation aircraft (g-akgj) added immediately behind the wing. the wing, engines, tail, nose, and cockpit were unchanged, but a new upper fuselage of 6 m diameter was added, giving the aircraft a "triple-bubble" appearance in front view. the entire rear section (including tail surfaces) was detachable to allow cargo to be loaded directly into the fuselage.