You can't remove the stanchion without removing the vent tube anyway. Your picture shows the tube going through the bulkhead between the salon
and the head
. Where it attaches to the stanchion is on the other side of that.
I'm guessin that MKII's have the molded toerail and the high-low stanchions.
Mine is a 1980. I googled around and found a pic of a 84's head
You can see the vent line in the rear left corner running up from the counter into the cabinet - the stanchion is right above that. (In a MK1).
From your pic, it runs through the cabinet in the salon
, through a hole in the bulkhead, and has got to be in the cabinet in the head. My need to look way up and feel for it to, or maybe there's a panel in the rear that can be removed.
All else fails, call Catalina
. They keep drawings and are very helpful on the phone