Okay, so I just confirmed that yes indeedy: a joystick extension cables also serves as an RGBi cable from the 128 to an Amiga monitor. Picture is really nice/crisp - but only in 128 mode. Does not work in C64 mode at all. So the RGBi port is *only* good for the 80 column mode of the 128 huh? Well that's rather limiting :-(
Yup - only for 128 mode.
The "hot setup" back then was to have a 1902, both ports connected at the same time, and switch between 40 and 80 column as needed.
