I've always called this the "shroud" Maybe that's just me
I've called it the 'outer skin' or outer cover(s).
The chassis, I always considered that as the piece inside that everything mounts to just like garys said.
If you look at a car, the chassis is basically what they call the frame which is also called the chassis by many in the industry. It holds everything including the fenders which are the outer covers or skin in this situation.
Most radio chassis are made from cast aluminum in quality radios (both commercial and amateur) while scanners and many consumer products are usually steel as they don't require the amount of detail that a commercial or amateur radio usually needs.
A few of the new uniden mobiles had two sets of covers or skins. One visible when fully assembled and then another set of covers underneath that which become exposed when you remove the outer covers for DIN mounting.
If you can find someone that did a DIN mount of a 536HP, you might be able to see if they did not throw away the outer covers are skins that are no longer used when DIN mounting.