Here are some pictures of mine before I bought a new main with a loose foot. BTW, this is not the original way the tack is connected. It was my jury rig. Here is a pic of the original rig from another boat.
My boat when first bought.
I don't use the furling boom. My main sheet jam cleat is actually on the end of the boom which doesn't allow the boom to spin. See here.
New main loose foot.
To answer your question, if you were to drill out the rivets that holds the cast aluminum end on the boom, you would probably find a nut fell off, or if there are no thread then the shaft bolt broke off in the boom.
Sooner or later you will probably abandon the furling boom due to all the problems you are discovering, make some modifications and just flake it on top of the boom.