Tubular to Adapter - Washer or Not

I've having a moment ....and can't recall.
I'm looking at the 1.5" adapter I picked up today and the nut that came with it is flanged as if it's a *built in washer* on the nut.

Do I just insert the tubular brass tailpiece into the adapter .......

With Nibco, for sure, the nuts which are included with traps and trap adapters have the slip joint washer molded into the nut.