Has anyone ever seen this?

It appears to be basically a miniature catalytic converter that surrounds the rear o2 sensor so that it only picks up on converted gasses...

http://www.bigdaddiesgarage.com/mini-cat-cel-fix.html

 

pretty sure you would get the same results out of modifying 2 stacked sparkplug non-foulers with one drilled out for the O2 tip clearance.
its basically the same exact thing but with some honeycomb material shoved in the end.