The transit screws are obvious, with burred caps.
I suspect the black screw (slightly recessed (countersunk) in to the casework?) probably anchors the brass chassis inside the 552, which is why it’s so heavy.
It might also be an earthing point?
Just added a pic…so probably I should screw it back in…?
Probably best. I’m sure Naim put it there for a reason 
Oh yes - I think other black boxes have the same and (I think) it’s an earthing screw, hence the serrated washer to ensure good contact with the casework and maintain the securing tension on the screw.
Ok… back in. No difference in the sound whatsoever !
Thanks
FYI, I think there was some thinking that with the earthing screws on other kit (the screws probably being dual-purpose) that scratching away some of the anodising around the reveal assisted with the earthing contact - I’ve never done this.
This screw (more a bolt) looks to be structural given the rounded head.
The serrated washer usually means it’s for earthing the sleeve to the chassis. In the hole where this fits on the sleeve you should probably see shiny raw aluminium. You don’t want to remove this one.
The transit bolts should have plastic knurled heads that are proud of the sleeve to allow their removal by hand.
I have several Naim boxes, which have a screw just like this. As @Richard.Dane says above, it to ‘bond’ the sleeve and chassis together. It wasn’t there on earlier Naim units - it was added at some point.
I have an older HiCap without it, and a slightly newer one with it. Both my 72 & 82 have it - and it has to come out to swap the Phono boards.
Put it back in. Tighten firmly. Enjoy…
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