I'm so happy that I kept a "service trench" behind my racks, so I can just peek behind that stuff easily whenever I need to change connections, or look something like this up
I have both, the older and newer d8b versions, and they both had the cover and the selector (one was black, one red).
I also looked at my HDR - there's a sticker over the selector that I didn't want to remove, but you can feel that there's a gap underneath, where the selector usually is.
On my MDR, the selector is 'exposed' right next to the power cable connector.
So, looks like they should all have it, but it might be covered up with a sticker or metal cover (single screw to remove).
But once again - I'm not sure if it's safe to simply switch this on the d8b. The HDR/MDR should probably be OK to switch over, since there aren't any extra power supply components in them, other than a standard computer power supply (at least not that I'm aware of... I didn't see anything that looked like it was an added power supply component).
So, my bet would be - fine to switch over HDR/MDR units. Unclear if it's safe to switch over d8b units, but they DO have a hidden (covered up) switch.