Mast Spreaders loose at base

3rd photo

I think they are meant to flex with the rig, meaning that rigid attachments might not be ideal.

I replaced all the rivets and the mounting plates are very solid now.
When I did this project, I happened onto "structural rivets" at the local Fastenal and realized that this is what the old rivets were that I was replacing.  They are much stronger than your basic pop rivets that they sell at the box stores.  Figured I'd mention this so others don't make the mistake I almost did of using standard pop rivets.

Use Duralac on *every* fastening going into an aluminium spar.   Use insulating gaskets under *every* stainless fitting  you have to remove or replace.

Its much better for the spreader and the socket to have some flexible packing between them as it reduces point loading, corrosion and fatigue.

My spreaders that have nearly identical sockets had badly degraded rubber liners in the sockets and rather more space to wobble than yours. I replaced them with collars cut from sanitation hose (which was the best match in thickness and resilience to the remains of the old liner) that was a press fit on the spreader end. I liberally smeared the spreader end with Duralac to prevent poultice corrosion before assembly.   It looks like your spreaders could benefit from some collars cut from bicycle inner tube as the gap you need to take up is quite a bit smaller.  *DONT* forget the Duralac!