Yep.
The 747, along with some other aircraft types, were designed to be able to externally ferry non-operative engines for urgent aog cases.
The 747 can take another 747 engine at a specific point under it's wing, as shown, in case one is needed urgently.
As the 747 engines are pretty large, only certain aircraft types can carry them to sick aircraft. So it can be most cost effective to ferry an engine on the wing.
The engine nacelle gets a cover put over it to reduce the drag caused by the spare engine. I believe it doesn't affect costs too much and is much cheaper than chartering a freighter aircraft specifically for the task.