Back in the 1960s and 1970s aircraft manufacturers devised special engine pods so that passenger jetliners could easily transport a spare engine between locations, often during the course of a schedule flight.
The first generation jetliners including the Boeing 707 and Douglas DC-8 were factory engineered for this special engine carrying capability. Special hard points were established on the underside of the wing between the fuselage and the inboard port side engine, and aerodynamically shaped engine transport pods could be easily and quickly anchored to the hard points.
Even the DC-10 and Boeing 747 (series 100 and 200) were engineered with this special feature. Photos of a four engined DC-10 are extremely rare, and fine engine 747s are not a lot easier to find.
In May of 1972, BOAC attached a fifth engine onto one of their then new 747-136 aircraft which positioned into Zurich’s Kloten Airport with the spare engine, which was possibly bound for locally based Swissair, which had just taken delivery of two brand new 747-257Bs of their own.
The photos below were taken of that special visit by a Swissair employee who kindly gave me a small number of slides he took back in the day. (He wishes to remain anonymous.)