Ex display Caple sink

Hi,
we bought an ex-display Caple ,stainless steel belfast style sink
It has the outer plastic part of the overflow and the screw, but nothing to go on the inside of the sink to put the screw through into the outer part?
The sink just has a rectangular hole in it for the waste
I can find the rectangular washer we need to go between the outer part and the outside of the sink, but what do we need on the inside to put the screw through to create the "sink sandwich"- or is that not how it works?