That portlight was made by Fuller Brush Company!
Probably back in the day of Mad Men
and the Graduate, when "Plastics" were relatively new and some odd companies were producing plastic products off all kinds.
What specific boat make, model, and year?
Why hasn't that been mentioned before?
While that portlight looks like it is deeply cracked and damaged due to age and UV...and I would not use it..
If the boat is worth restoring...
I would consider adding some stainless portlights
. Give your boat some bling!
316 Stainless Steel Standard