For transportation of spare engines, early 747s could accommodate a non-functioning fifth-pod engine... Carrying #cargo internally is always more efficient but the fuel cost due to drag from carrying an engine like this is sometimes less than the cost of hiring a cargo plane to haul a spare engine somewhere to where there is already a short-hop passenger flight going - especially if there is some urgency involved.
A few explanations:
The special wing with fitfh-engine attachment was standard on that 747 subtype - before the advent of wide-body airliners with cargo doors that nowadays can easily accommodate a high-bypass turbofan engine. On the #Vickers VC-10, the configuration for ferrying a fifth engine required a mounting frame - much less elegant: ▻http://www.vc10.net/Technical/oddities.html (bottom of the page)
Detail of the optional fan cover and mounting pod:
And for the fifth-engine fans (I know you’re out there !) here is another entirely different way to mount a fifth engine on a 747: