The porthole's ambiguous appearance teases the viewer to question its purpose; when in fact its purpose is pure and simple.
One of the 7 x 7 inch circular glass panels is held on by a removable Allen bolt to contain whatever you feel necessary.
Here's another sexy shot.
Crucial Detail sells these babies from Europe and the US for $99 not inc shipping.
Have a look for yourself: http://crucialdetail.com/work/porthole.html