Spoiler:
Wait, I think I know what's going on. There is a series of teleport passageways behind the rock that brings you to a switch
https://imgur.com/HXSzFo1. If you activate it, you get to fight the 'Enhanced' Ryujin-sama. If you flip the switch again, or don't the first time, you fight regular LV2400 Ryujin. The higher level version is not recorded in the bestiary.
I knew something about the level was off. I guess that explains how you get additional copies of the item, and based on the next post, apparently a few others as well.
That's neat because it provides a more tangible goal for those who want to massively overplay the game utilizing the infinite corridor. At first I thought it sounded like a big pain to go through so many IC opening events plunging that deep, but I realized that after you're done gathering most kinds of items (other than special boss drops and additional awakening items) there's very little reason to open rank 1~2 or possibly even rank 3 chests, so you don't need to deal with as much tedium from the item-opening cutscene and flashes. Even the 4~5 chests would mostly be for the chance of 20 infinity gems, and you generally don't even see one of those per floor. I also wouldn't need to worry about getting 40~50 floor chains once I have enough tokugawa statues... yeah, it all gets easier.
As for my current progress, completed B11f and the right-side quadrant of endgame 30f.
Gems sure do rain down like candy grinding in the depths of b11f though. It actually makes it look completely reasonable and realistic to max the entire party's gems once you're overleveled enough to speedgrind, esp. with Rinno in front and nazrin in back for drop boost. Jewels though... oomph. Enough to get the all more important ones done, at least.
Edit:I used tomes to compare Reisen to Miko, after considering that even the likes of Miko is best off with a quartz charm now... and if I really do come up to lv3000+... well. The problem is most of her current stat pool comes from equipment (taking off ONE piece of gear HALVES her max hp! and it's not a first aid kit!), and the amount of stats off equipment will scale down significantly over time with levels and library. Unlike Nitori, who has a massive stat headstart and others catch up, Miko is mostly on-par with others, just better affinities, and then will fall behind later instead of falling back to normal levels. A Reisen tweaked as heavily as her is already about as good with better status support at only lv~1700, and I can peek at later scaling by removing all gear but their tokugawa statues, at which point Reisen blows her utterly out of the water- even accounting for moderate equipment boosts and her passive benefits there's no way Miko will keep up. Miko does still boast higher affinity potential and a dual-element debuff and support heal (which currently fullheals any non-tank- not bad) but I guess I just don't want to reach lv3000 and THEN swap characters so then Reisen's ends up with a 10% exp deficiency mega late in the game. Miko is excellent if you're just planning on getting to corridor 640, beating True Final Boss, and stopping, though.
Unfortunate that Kaguya isn't a more worthwhile full-time party member, though. Full family bonus would be nice. Already using Eirin. In mega endgame her delays eventually aren't a problem and Eirin can overheal her, so it's workable, but there's not much reason to when you aren't fighting the def/mnd monoliths.