Prior to this last round of pictures I had *no idea* what you were talking about, you hadn't posted any of the outer shroud. (I'd assumed that everything was missing, not just the bezel.) My first thought upon seeing that notch was perhaps there had been a locking device in that corner that someone had hacked out to open the case, but it does look like the edges of it have been turned/finished. That certainly implies this case was being used for *something* odd, but it doesn't negate the near statistical certainty that the case is a completely off-the-shelf item ordered out of an OEM catalog and modified by a the reseller/end user.
Looks like a bone-standard-if-somewhat-bland early-ish Pentium 4-era side panel to me. (The case is white and plain, which in my mind that anchors it to the early 2000s; colors like silver and black started being popular enough with OEM/small builder/home-built machines around 2003-2004 to start outnumbering the white/beige boxes, even for systems built for business/industrial use.)
Anyway. It's impossible to prove a negative *and* that cut-out corner is "interesting" so the case may well have served time in a lab setting, possibly as home to something interesting. (Or driving something interesting.) But it's just as likely that the case sat for years collecting dust, I dunno, under a cash register or in a stockroom and that cutout was added to route a cable or to clamp the thing into a kiosk. The missing drive rails and bezel suggest it was brutally stripped when it was removed from service; that *is* the sort of thing that happens to machines in commercial/industrial settings, which I guess is a point in favor of it not just being the box from under an accountant's desk. There might be an interesting story behind how the Mac drive tray found its way into it, but I would have some pretty serious doubts about it having anything to do with IBM/CHRP/etc, etc.