I think the BJC site has a few old versions, but probably duplicates of these ones.
The "~bh" ones are where the snap.berkeley.edu home page lived (yeah, really served from my directory) until Michael and Bernat made the new server for the site and the cloud back end.
Any version of Snap! that was in the BJC site was an exact mirror of a version on the Snap! site. I don't think you have to worry about them. Same for the ~harveyb ones.
Umm, well, I'm not sure where you'd find old BJC ones, the whole idea was that there would be a single URL that would mirror the latest version. And we wouldn't back up the mirror because we do the backing up on the Snap! site!
I've wondered if versions other than previous (/versions/previous), dev (/versions/dev), micheal-dev (/versions/micheal-dev), and now also v7 (/versions/v7) are available.
(a) I don't know, and (b) that would be subject to change anyway. There will always be the current version, and some recent older version that we consider stable, and there will usually be a dev version.
Barring something unusual, there's always going to be a dev version. It's just that on the release of a stable version, the dev version is a duplicate.