[pavlov]

But that came after 1280x1024 had already been a popular resolution for a decade.

What was the original 5:4 CRT that gave us this resolution?

[TylerE]

There isn't one. Running a non 4:3 CRT mode simply resulted in a change in aspect ratio. However, by adjusting the geometry in the settings menu get to something closer to the aspect ratio by using the marginal areas of the tube. Pincushion will be a bit worse, but who cares,