Okay, I filled in KCS-009 as Keys of the City. A seller on amazon.co.jp was selling a "Keys of the City" album described as KCS-009 in the descrption (I bought the album, so the sale is gone). But that's pretty confirmatory.
As for the last one, KCS-023, I still have no idea. My official guess is that KCS-023 was supposed to be for Northern Verse, which was cancelled:
https://vgmdb.net/album/14356
and that the album was thus never printed at all.