Bath Store Shower Waste Trap Issue

I have been looking at the waste trap supplied as standard with a Bath Store shower tray. I have been helping install some Bath Store kit.
The outlet of the trap has a black item inserted which fits via a compression fitting and appears to to be a converter from a narrower diameter to a wider one.
However, neither the "narrow" end is 32mm of the "wide" end is 40mm. Does anyone know what the true dimensions are? What will they fit?
More to the point: how can this item be connected to regular 40mm piping? Ideas, opinions, please
If this isn't fixable, then some questions:
What are these traps? Where does Bath Store get them from?
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