Those are hp/compaq e-pc's
neat little boxes. We actually used em as print servers for a few years. Though they are flakey at best. The first round of them had a major mobo problem that required a full replacement. which was about an hour process.
Yes, it's a lock
Yes you can "bust it off" but you have to already have it open to do so without damaging the case
It SHOULD only lock the hdd tray and compartment. though it might lock the "sleeve" case from coming off.. been a while since I had to take one apart.