Greetings,
Mr. FB. I've dealt with the same issue on our vessel (non GB) in that the stanchion bases are attached with machine screws with no visible bolts underneath. Now whether or not there were
ever bolts underneath OR the fiber glass was simply tapped is unknown. In MY situation I purchased threaded inserts similar to this:
In hindsight I would have installed them differently (used a large slotted screwdriver which didn't work all that well). Next time I'll use something similar to this: