What is this?

It's supposed to represent a deadlight on the hatch of a late version of an 80' ELCO PT engineroom access hatch/vent.  The real deadlight was flat.  This is from the parts manual for the 80' ELCO:

Al Ross

results may vary wrote:

Thanks.  I could only find one pic that showed it and it was flat.  Not to sound like an idiot or anything but, what is a deadlight?

Essentially, it's a window, normally with a shutter or some other form of curtain that can be closed to prevent interior lights showing through the glazing.  The dark angular object on the far left of this photo is the port deadlight cover on the inside of the charthouse.

Al Ross 

results may vary wrote:

what is a deadlight?

A "dead" light (or lite in the sense of "pane") is one with an obscure cover, one that cannot be seen through.  It can also mean an opening with fixed (inoperable) glazing.  The light is "dead" in that it cannot move.  Sailors might say that if you were trapped behind one, you might be dead, since it will not open.

The solid cover for a "port hole" is called a deadlight as well.