HIN that starts out with the two letters of a state followed by the letter "Z" indicate that the original HIN (if it existed) is missing and the state issued a HIN. "FLZ" is a HIN issued by the State of Florida. States have this process for any registrable vehicles. Boats and trailers are the most common whether home-made, one-off, or production.
Technically it does not mean "home made", it just means that the original HIN is not found. If it was built before 1972, it will not have a HIN, but would have a "hull number".
It is possible to have a boat with a state issued HIN and then locate the original HIN that was buried under layers of paint or from the secondary location. If the vehicle/vessel is inspected by the authorities, it could be registered with the original HIN.
I may sound nit-picking to some, but calling a production boat "home-made" just doesn't set right either even though the term is widely used.