Hi Ben

The round shaft of the rudder stock ( galvanised steel on early boats, and at sometime upgraded to stainless), has several flat bars (approx 1” x 1/4”) welded to the rod, extending aft, about 2/3 of the fore and aft length of the rudder blade.
The two half shells of the rudder blade were first laminated  separately, and before being taken out of the moulds, the rudder stock, with the prongs was laid into place in one of the half mouldings, and several layers of grp laminate bonded the prongs into one of the half shells.
When that was sufficiently cured, the two moulds, (with the two shells still in place), were firmly clamped together, with polyester resin glue paste around the edge.
Next, the whole thing (still clamped), was put in a vertical position, ( rudder stock shaft up),  a hole was drilled in the top edge of the rudder blade, and a mixture of slow-catalysed resin with microsphere glass packing powder was poured in to the fill up the space between the two halves of the rudder blade. When that was thoroughly cured, the rudder was removed from the moulds, and the joint between the two halves (and the hole where the filler resin was poured in) reworked with gelcoat to make the whole thing waterproof.

If you can detect rotation of the shaft in the rudder blade, then it would suggest that there is a failure inside, either of the welds of the prongs to the shaft, or the lamination of the prongs to the shell, with collateral damage to the filler between the two halves. I am not certain that the microsphere/resin filler technique was used in earlier production.

Hope that that answers your questions. Good luck with the repairs.

Cheers

Bob