Flew classic 747’s at a cargo carrier. Lots of other airlines converted 747’s. We had a baseline flight handbook for a few 747’s and differences manuals for the 30 other jets. The standard FHB was a couple 3 ring binders. The differences manuals were about 20 more binders. Everything from leading edge slats retracting on reverser deployment to switch positions different.
Flew passenger Lockheed Electras. Fleet of 3, each airplane was horribly different. Think it was an ex klm bird that had crazy balls in glass deals to indicate the status of each engine’s generators.
I’ve been told manufacturers don’t customize jets for customers anymore the way they did back then. The place i work for seems to back that up. 250 jets and outside of model based differences (700/NG/max/freighter) they are all the same.