If you type ‘Portlights’ into the search box you will see there have been various threads about this topic in the past. The threads from last year and 2 years ago are probably the most relevant.
John Bird found that Eagle windows (www.eagleboatwindows.co.uk) have taken over the manufacture of these lenses which are a non standard Lewmar size. To complicate matters further I think there is a Mark 1, 2 and 2a variations. I have not used Eagle windows but I believe they are very helpful and should be able to identify which lens you have.