as an ex field engineer, I suspect you are confused... the sides of these were solid sheet-metal, with front-to-rear ventilation.

I work as a field engineer for a large unix systems company (also lots of databases and such)... spend most of my days swapping FRUs (field replaceable units). HP, Sun, IBM all work the same way. Most parts are a single board or at most 2-3 bolted together. We very, very rarely have to pull a whole 1U system (easier/cheaper to replace the system board).