It should be a standard stuffing tube packing gland with gasket material inserted at the top, similar to the stuffing tube that you
propeller shaft runs through. If made properly there would be a Babbitt bearing at the top and the bottom. With the packing gland just above the top one. And a thrust bearing is above the packing gland a couple of inches for the rudder post to hang from if you have a spade rudder. All of this comes in all the various designs as there are leaves on a tree. The rudder post should have a screw hole at the top for an eye-bolt so you can hang the rudder from it as you change out the top thrust bearing. The packing gland should be reasonably accessible for repacking. But then some builders seem to think that we have very tiny elves on board to do all of the maintenance on the boat.
I have changed out a few packing glands on the shaft and the rubber posts. The packing gland should be greased thoroughly and do a weekly PM on it when doing heavy sailing. And expect to get very greasy when doing such maintenance.
PM = Preventive Maintenance