Almost certainly you have the 'Select' mortise lock fitted to my FP Maldives
and FP Athena. They are not very well made and can fall apart inside which in my case required me to break into my own boat, plus getting keys for them can be a problem. Never have found who made them. I have found what I think would be a replacement, made of stainless, but it looks a bit bigger. My plan is to make a escutcheon plate to go on the outside from stainless which would hide the old mounting and cylinder holes.
If you have the dimensions of the existing lock that would be useful