5th pod
IIRC the whole thing was developed by BOAC/Qantas-in the 707 era as VCness has mentioned)-and they probably still get royalties (they certainly used to) for each engine carried whoever is carrying it.
I believe nowadays that you can (just) get a big fan engine into a wide bodied freighter and so the practice isn't as prevalent as it once was.
To adjust the performance of the a/c carrying the fifth pod the word 'FITTED' was inserted into the FMS to adjust the performance details that were required.
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