Rear Entry Fill Valve.

I have fitted quite a few side and bottom entry toilets but this is my first rear entry type.

I am trying to fit a rear entry filler valve to a noken compact cistern, it has a gerberit filler with brass thread but there is no nut or anything to fit the valve to something. there are no holes in the cistern apart from a cutout at the back, so my question is should there be a retaining nut and do you attach to the rear stud partition and it just sits in the cistern.

Thanks In Advance.

 

Its one of these but the flush mechanism does not have a clip and its a single upright part.