Here is the only reference photo I've found that shows either one or several rectangular, square or bracket parts that placed on top of the "switch" hole:
Unfortunately, the photo has a very low resolution so it's difficult to tell exactly what was added and all the reference photos or stills taken after that early shot (Leia on Hoth in the command center) clearly show a hole in the "switch" part. So if there was something originally there, it fell off early during the filming.
The "radio" or "conical" knobs appear to have fallen off the prop just after filming wrapped because the People magazine photo shoot shows the prop without these knobs and of course this is how the prop remained during the museum tours.
While studying the early still, I did notice a couple of holes in what looked like a rectangular piece and came up with an idea. Since several the props used in ESB were dressed up with V8 motor engine parts, maybe the original part was a motor/coil:
A black and white still has a nice side view of this prop
, but it doesn't show the raised holes in the motor/coil so it's possible that they flipped it over to seeing the text:
Was this what the original prop was supposed to look like? Personally I don't like it with the motor/coil part, but who knows.
To add more to the mystery of the missing parts, I came across something in one of the museum photos.
It's fairly clear in the photo below that this is residual glue left from something. Was it a mistake that was removed or was there something here at one point that fell off? Was it another knurled piece?
Also, notice the two red marks on either side of the hole. It's possible this is where the rectangular piece was glued. Either that or this is just wear and tear.
