Thrust Bearings:
There was no reference in the Aft Transmission Build Manuals/Drawings about which way around the Input Thrust Bearings should be installed. (IIRC it should have been to hide the Ser No's from view) although Boeing had experienced some failures previously and attributed them to that reason.
It is thought that some "old sweats" at Almondbank recognised the issue, figured out the requirement, and jotted down a note to put them the right way around, which they did perhaps for years. Then somebody else was given the job and did not read that particular "hint". The Bearings were installed with no particular favourite setting.
It was a complete fluke that both aircraft flew on consecutive days, 8000 miles apart, as the boxes had been built some time apart. And it was also a complete fluke that no-one was hurt significantly.
From memory, only those two Xmsn's and one other had the bearings incorrectly set. (I may be wrong - it's been a while)
In Almondbank's defence Quality Assurance was in its infancy then and this sort of (one-for-the-notebook) attitude was rife - I did it myself; I saw it happen around me; and learned from it.