I already said in my opinion the case chassis itself dated to somewhere around 1999 and was updated to add the side fan sometime later than that. That may actually be a late date, I'm basing that on the fact that the system I had which I *swear* was exactly that case was built sometime in late 1999-2000. So far as I know the tooling for parts of that case could date back to the original release of the ATX specs by Intel in 1995.
In any case, it wasn't particularly uncommon to see cases of around that vintage that offered the possibility of mounting either AT or ATX-style motherboards. (And conversely I've seen motherboards that could be mounted in either style of case; the motherboard would be roughly square and the ATX mounting was generally a nasty kludge job with cables running to a backplate. Of course, by this point AT motherboards were *all* nasty kludge jobs with wires running from all over the motherboard to ports mounted on slot covers.) You'd have to swap out the back panel to accommodate an AT board but the tray part of the assembly could have been used in multiple case configurations.
If it *really* was something modified prior to Y2K for something that needed a special fan layout I'd actually say a far more likely possibility than anything CHRP would be an Alpha motherboard. Alpha was HOT in the late 90's, both literally and figuratively, and ATX incarnations weren't rare.
That bizarre adjustable dingus to hold the cards in place (in addition to the slot cover screws) is by far the most interesting part of the case; again, I swear I had almost exactly that case but it didn't have that part. That actually makes me wonder if this might have been mounted in a truck/van, or some other location where vibration might be a problem. (Inside a rolling kiosk? On a factory floor or inside a piece of manufacturing equipment? Etc, etc.)