I'm with
Kimberly and
Snopboy -- the lowest client version that's publicly available or existent is KMS Beta v1.23 (2001/2002). This client, unlike the rest, uses their Alpha UI. One of these days I'll release OrionAlpha, which is my (almost) 100% complete emulation of this version. I just keep forgetting about it.. Also, while you don't see any servers of this here on GTOP, they do exist in the Korean PS community. There was a server running KMS v1.23 for a while called OldMaple or something, forgot the name.
EDIT: Although, as far as publicly open-sourced is concerned, v40 Beta Edition is still technically "lower" than v12.