I had the same problem in both of the Gramps I owned. In each case it was a worn hole in the tiller head at the top of the rudder post.
The shaft comes up from the tube and the rudder is held in place with a cross pin that eventually wears a hole in the deck
as the rudder wieght wears away the glass. This is a primary entry point for water
into the plywood core
of the cockpit
sole. (also a grampian fail point) Than can be fixed with the steel
plate mentioned above.
Meanwhile the tiller head, made of bronze is wearing. At some point you can over drill and sleeve the bolt hole but I'd be careful, there isn't much meat there. It might be better to remove the tiller head and have both holes brazed full and new holes drilled. The same applies to the tabs on the tiller itself. Taking the tiller head off is as easy as loosening the cross bolt and levering it up with a crow bar using a plate to keep the deck
clean and dent free.