Yes, I realize there are 3 velcro strips around the tub for the cover, but that black rectangular piece is what I was looking at. 

I guess I DID get a picture of it inadvertently during my dismantling.  It was on the opposite side of the one on the gray car.  So, there's supposed to be 2 of them, I think.

 

 

 

 

It turns out it is part of a velcro system for holding the carpet in place.  Here's a shot of some original carpet with the velcro attache.