1997 PC-9801 series visual novel. https://vndb.org/v9077
Reminder: New tag rules say this is not to be classified as "Misc" or tagged as game manual, since it is pornographic in nature. https://forums.e-hentai.org/index.php?showtopic=275346

This visual novel has one of the first 'futanari' CG sets I think I ever remember seeing. Pretty cool stuff. When I ended up getting a physical copy, i decided I'd try to scan in some material that I didn't see online yet. I also have the game box in very nice condition, so maybe I'll update this with scans of that in the future.

It also has transgender characters, cross dressers (to the extent that those differ), and other things that rather rarely appear in other VN's from the period. It's mostly a silly game though.

For the CG from this game, see this gallery:
https://e-hentai.org/g/576682/f1bd65dbfd/

There was also a Windows release, which is a little bit less pixellated, if that's more to your taste.
https://e-hentai.org/g/283717/83a0fb9981/

It's not self-published (by the artist, anyway), and the manual is not digital, so going by the flowchart I guess this goes in "Manga" (even though it feels kind of wrong)? - But I'm doing "Doujinshi" because Space Project was both the developer and the publisher of the game box... I've asked for clarification on category in the meantime.
Published by Space Project Co. Ltd., but spelled "SPASE PROJECT" on back of the box.
It's not a "Game CG" because it's not digital and the pages do not appear in the game.

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The PC-98 game itself, if anyone is interested:
You can find the PC-98 game disk dumps on archive.org, currently; someone else (a hero) copied and uploaded it there.
https://archive.org/details/seiyoku-gakuen-seraphita
If you want to play these on an emulator, or write them to disks without a "Greaseweazle" floppy drive controller, you will need to convert the disk images first. The "Greaseweazle" command line program] can do this:
[code]gw convert disk-file.scp --format pc98.2hd disk-file-converted.img[/code]
https://github.com/keirf/greaseweazle/releases
If you want to play them on a PC-98, the PC-98 is somewhat special in that its 3.5" high density floppy disks are usually 1.2MB, the same as 5.25" disks are, and they are both formatted identically apart from their physical sizes. So that person's 5.25" disk copies should be writable to 3.5" disks without problems, if you have a drive that can spin 3.5" floppies at 360 RPM. Many USB floppy drives let you do this, if you first make 'img' files like above.

There is also a rip in a different format (easier for emulators, but less "preservation-grade") in the "Neo Kobe" archive on archive.org in the 'doujinshi' zip file, under "Space Project."
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The Windows version:
I do not have a copy of the manual or box or a physical CD for the Windows version, but if you would like a good CD rip of the game, let me know on the forums in a PM and maybe I can get you sorted.