H25 rudder
I just looked at mine today (before I'd even seen your post, rwittenburg!). A while ago I had lifted up and rebedded the fiberglass plate around the deck because it leaked and the rudder never fell out, so I was left wondering till now. It has a collar about 5/8" high round the top of the rudder shaft like a kind of bearing under the tiller fitting. I discovered that it seems to be ONE sheet-metal screw through the collar into the rudder post (and mine looks like it's half sticking out). As far as I know that's what holds it up.
The '78 ('Mk 2' boat) is a little different in that your rudder stock stands up higher off the cockpit seat than mine does. I don't know why that was done (mine looks cooler-- and is probably stronger). But that might be the the 5 inches you mention. If your tiller head is within an inch or two of the plate on the deck/seat then you may be missing the collar or the screw for it. I would rig a tackle to catch the rudder should it drop out and then fix this out of the water as soon as you can. (Please don't ask me where to get a new rudder. I could make a mold off of mine and build a new one... but it'll cost ya!

)
I already decided I will prop up the rudder, remove the tiller head, and drill out the collar and the shaft, straight through, and install a 1/8" clevis pin. I don't like how it appears to be fastened now and though the rudder assembly is the one thing on the whole boat that has never given trouble, I am going to improve it. (This is what I do when I have so much money and time on my hands... no, wait. I don't have either. I just do it anyway.)