5th Pod
I think it was a standard Boeing build to put fittings in the inboard left wing for the extra engine. I have fitted several, & the other point we had to sort out was the fuel loading which was non standard, with balancing fuel in the right wing.
As a point of interest I was also involved in fitting a 4th pod on a TriStar, but that went on the inboard right wing.
Tristar 500
Edited for spelling!!