You are quite correct. There is a page on this topic on the website Westerly Oceanmaster 48 chainplatesYes had similar on a friend's Westerly but with A bolt type of shroud attachments
First indication that something was wrong was finding the bit with two nuts on the cabin floor. Then I spotted one of the shroud anchors lifting from the deck. Subsequently one leg of a second one sheared as we tried to undo it for inspection and a third was severely wasted. The fourth was OK I guess because I had taken it off , refitted and resealed it several years previously because it leaked into the locker where I kept my stuff when on board.
I think your failure is due to stress corrosion but I am not a metallurgist ..... Vyv Cox will properly diagnose when he sees it. Ours was, I am sure, due to crevice corrosion as the fittings were severely wasted where they passed through the deck.
You are coming up with some seriously scary stuff! I hope your original problem is getting solved and it would be good to hear the solution to that one. I have added some bolts to my list for winter checking. Thanks for sharing.You really have to work at it to get access - what a horrible design, where these essential stressed members are deeply concealed.
To be clear, I don't have a Westerly, this really is a friend's boat.You are coming up with some seriously scary stuff! I hope your original problem is getting solved and it would be good to hear the solution to that one. I have added some bolts to my list for winter checking. Thanks for sharing.