I randomly spotted the manga´s new and additional Seinen tag when I set the anime endpoint and then started to dig down and indeed saw that it is being listed as Part of the Young Animal Line (Berserk, March Comes in Like a Lion) on Japanese webshops. This is how I understand it:
YA´s imprint publishes in a print size that Hakusensha doesn´t use for female publications so a few of those get printed or reprinted in the YA line to utilize its print format. Does that make it or Oz´s bound republication though YA "Seinen". Lord knows... Did I get this right?

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So it's not actually seinen? Doesn't give the vibe either. I was surprised to see the tag. I don't think the target demographic are men.

The mag Oooku ran in was a Shoujo publication until this manga´s tail end was reached. I don´t know if Ooku was still in the magazine when it went Josei but I think so. Vol 19 still features Furigana, a dead giveaway of a youth demographic but I will admit that JoJo still uses Furigana to this very day and I saw a few "native" Seinen or Josei manga with it here and there. Again, just a few and JoJo Part 8 ain´t for kids in any way, I´ll tell you that much. Furigana = below 18 readers in nearly all cases. But all of us saw our share of near hentai and gore porn in Shounen mags...

All in all, this is a mess. The manga was a regular Shoujo in a surprise to most foreigners despite its R-rated content and bleak nature (Nana and more or less X/1999 have the same issue) for most of its run but now has a partial claim to 2 other demographics.

Anyway, would a fan of Berserk get something out of Ooku now that both share ad space in the end of the volume listings? Yes.

Edit: The TV-Ma Netflix anime got an R18+ in Japan. Berserk films 1 and 2 are R15+ and only the version of film 3 without the mosaics is R18+. There is an R15+ cut that cuts nothing, you just have to live with genitalia pixelation. Amazing work Eirin. There are no ratings boards for books and comics in most of the world so the print version is All Ages and the basically panel-by-panel anime is 18+.

Last edited by residentgrigo at 1:40 pm, Oct 8 2023

________________
I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb