So the way they tended to do the 90s ones from what I have seen is they would glue a piece of aluminum to the underside of the hull, drill mounting holes through the hull and aluminum, and there's your mount.
On my parts boat a lot of these metal backing plates have come unglued, this happened with the bow light backing plate as well. Yes you can thread things like this into fiberglass ok but there's a reason they don't do this from the factory so I try not to do it. I sure would not trust a cleat threaded into fiberglass only!