Rusted out zirk or something else?

Hiding behind our nonfunctional hot water heater at the base of the rudder post "sleave" is what is now a rust plug (see attached photo). My Ericson 30 had a zirk on the rudder post "sleave" (what is this called?). I don't see one on our 1982 38. Does anyone have knowledge about this? Was this a zirk? It is just below the waterline. Would a zirk be below the waterline? Given that it was behind the water heater, if it was a zirk, it has probably not seen grease since the heater was installed (44 years ago?). If not a zirk then what? It has shown no sign of leaking so far. But the rust is slowly crumbling away. It would be nice to know what to do when I haul out to address it.

Yeah, that looks like a rusted zerk fitting. I would drill it out, clean the area, add some epoxy if needed, then thread in a new zerk. And apply grease once or twice per year to keep the rudder turning easily inside the rudder tube.
If you look carefully you might also find a second zerk above that one near the cockpit floor underside. It should be greased as well.
Frank

Thank you, Frank.