Well I took everything apart, including removing the rudder completely, and can provide some updates.
The report from Peter Richardson of Toroa seems to match approximately the shop drawing on the web site, which shows 2" (~50mm) distance from the top of the keyway to the top of the 2.375" diameter portion of the rudder stock. On my Amadeus, the top of the keyway ends about 4" below this, and in the bottom of the keyway occurs about 1" INSIDE the rudder tube, so it looks as though it was milled offset from the drawing. This can be seen in a photo of the rudder stock after removal; the shiny section is below the top of the rudder tube.
I measure the rudder tube outside height to be 10” from the top to where the filleting and cloth of the bilge meet the tube. Measured with a tape measure hooked onto the top and measuring underneath the boat shows 14”.
The shop drawing for the Nonsuch 26 available on the INA site says that that rudder tube is nylon, which was confirmed by Bob Cole of New Max (N26). I do not believe this to be the case in my N30U Amadeus. Damage seen inside the tube, and viewing the tube from the top suggests a laid up construction (ie. fibreglass).
I note the workmanship of the aft gusset shown here is rough, and continues this way downwards. There may have been a repair in the past? The tube thickness is about 1/2”, as I believe I mentioned earlier, the thickness measured underneath the boat where it emerges is less than half that.