Have you examined the disk image with, e.g. Apple Commander? The installer reads the volume name, not the file name. Might be corrupted or the wrong image.
The image shows up mounted on the desktop with the right volume name, Pascal.Extras1 (the file name is different but that shouldn't matter as you point out). It also shows up in gnome and orca shell as correctly named.
Its almost as if the installer will only look at a volume in certain physical drive (Slot 5 Drive 1) even though the installer itself was run from a disk image mounted from Emu.
Maybe the Emu image is not seen by ProDOS (GS/OS) as a floppy drive, its seen as a hard drive (Not sure if that even makes sense)
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