I was on my way to Southeast Asia back in the early 70s and was sitting in the bar at SFO sucking down my third scotch when I noticed a 747 on the gate. For some reason it looked a bit odd and then I noticed the 5th engine. Only after confirming it with someone more sober than me did I believe what I was seeing. And later I found that yes, a number of airlines had 'hard points' under the left wing to ferry an engine. Looked quite odd..