During the development of the Boeing 747SP, a combination passenger-cargo version called the SP Combi was proposed. With a large main deck cargo door in the aft fuselage, the SP Combi was billed as the ideal replacement for the aging fleet of Boeing 707 and Douglas DC-8 aircraft as it could fly 186 passengers and nearly 65,000 lbs of cargo farther and with better economics than a 707-320C with 147 passengers/6,800 lbs of cargo or a 707-320C freighter with 58,000 lbs of cargo.
The SP Combi's 328 seat all-passenger configuration could be quickly converted over to either a 242-passenger/four-pallet arrangement or a 186-passenger/eight-pallet arrangement. To test the idea, a China Airlines 747SP was used to test the accessibility of the main deck cargo door with standard main deck loading equipment given that the 747SP's wing trailing edge was much closer to the aft fuselage location of the proposed door than on the full size 747 Combi aircraft. Black tape was used to outline the door location and tests were successful. However, the airlines showed little interest and no SP Combis were produced.
Source: Boeing 747SP by Brian Baum, Great Airliners Series, Volume 3. World Transport Press, 1997, p41.