Concealed shower valve questions

[Evren]

Hi All ,
I am trying to fit a concealed shower valve into a stud wall which i have access to the rear of only.

I have all the bits that came with the system (2nd hand ) but quiet confused as to how the front decorative panel gets secured in its place.

Made this plywood template to act as the tile to see how it all comes together.
The threaded bars screw on to the valve hence grab the tile which is all good however after that the decorative panel just slides on freely as well as the
chrome rings that are able to slide in and out freely which does not make any sense.

Also chrome rings have rubber O rings on the inside where they slide over the threaded bars.
Are they meant to have O rings on the outside as well as i can see they have grooves in them as per photos.

 

[thomp1983]

Others I've seen normally have o rings in the grooves on the chrome rings that wedge it in place

 

[Evren]

Others I've seen normally have o rings in the grooves on the chrome rings that wedge it in place

So I must be missing the outer O rings.

In that case am I right thinking everything behind the panel has to be water proofed and any water getting behind the panel should just run down and out ?