Ah that looks like its a plugin module for those multipurpose frames and its usually the frame that has the power button.
They likely ran out of room for a power switch. Some of the frames i have seen with multiple slots use the spacing between the slots to squeeze in a really narrow power button, its usually colored bright red so that you notice its not part of the frame. If they had to add a normal power switch on these frames that would make the whole frame a good 15mm wider for nothing other than the power button (Tho i suppose they could also have some GPIB LEDs above it to use up the rest of the space.)
Have a look inside maybe there is enough room behind the front panel to put a power switch in a corner, perhaps replace that grounding post with a toggle switch.