Ho hum! I assume by a deadlight you mean the hinged metal flap that can be secured across an opening porthole?
These are ship equipment, not yacht equipment, and I would seriously doubt whether yacht size portholes ever had them. They are a sort of freak effect of the Load Line Rules.
Mirelle, I might not be using the correct term or rather using the American term. What I need is a porthole that doesn't open! I have one in my wooden forehatch which persistently leaks because it is incomplete (which I could explain but would take a long while)